Remove the compat30Native project from sbt

The build currently has a `compat30Native` project which causes problems
in the IntelliJ project import (through bsp)

```
sbt:scala-collection-compat> projects
[info] In file:/Users/luc/scala/scala-collection-compat/
...
[info] 	   compat213
[info] 	   compat213JS
[info] 	   compat213Native
[info] 	   compat30
[info] 	   compat30JS
[info] 	   compat30Native
...
```

Author: lrytz

build.sbt

@@ -54,7 +54,7 @@ lazy val scala212 = "2.12.15"
 lazy val scala213 = "2.13.6"
 lazy val scala30  = "3.0.2"
 
-lazy val compat = MultiScalaCrossProject(JSPlatform, JVMPlatform, NativePlatform)(
+lazy val compat = new MultiScalaCrossProject(
   "compat",
   _.settings(ScalaModulePlugin.scalaModuleSettings)
     .settings(commonSettings)
@@ -85,7 +85,8 @@ lazy val compat = MultiScalaCrossProject(JSPlatform, JVMPlatform, NativePlatform
         )
       },
     )
-    .jsSettings(
+    .disablePlugins(ScalafixPlugin),
+  _.jsSettings(
       scalacOptions ++= {
         val x = (LocalRootProject / baseDirectory).value.toURI.toString
         val y = "https://raw.githubusercontent.com/scala/scala-collection-compat/" + sys.process
@@ -100,9 +101,8 @@ lazy val compat = MultiScalaCrossProject(JSPlatform, JVMPlatform, NativePlatform
       },
       Test / fork := false // Scala.js cannot run forked tests
     )
-    .jsEnablePlugins(ScalaJSJUnitPlugin)
-    .disablePlugins(ScalafixPlugin)
-    .nativeSettings(
+    .jsEnablePlugins(ScalaJSJUnitPlugin),
+  _.nativeSettings(
       nativeLinkStubs := true,
       addCompilerPlugin(
         "org.scala-native" % "junit-plugin" % nativeVersion cross CrossVersion.full
@@ -112,10 +112,10 @@ lazy val compat = MultiScalaCrossProject(JSPlatform, JVMPlatform, NativePlatform
     )
 )
 
-val compat211 = compat(scala211)
-val compat212 = compat(scala212)
-val compat213 = compat(scala213)
-val compat30  = compat(scala30)
+val compat211 = compat(Seq(JSPlatform, JVMPlatform, NativePlatform), scala211)
+val compat212 = compat(Seq(JSPlatform, JVMPlatform, NativePlatform), scala212)
+val compat213 = compat(Seq(JSPlatform, JVMPlatform, NativePlatform), scala213)
+val compat30  = compat(Seq(JSPlatform, JVMPlatform), scala30)
 
 lazy val compat211JVM    = compat211.jvm
 lazy val compat211JS     = compat211.js

project/MultiScalaProject.scala

@@ -59,39 +59,33 @@ trait MultiScala {
   }
 }
 
-object MultiScalaCrossProject {
-  def apply(platforms: Platform*)(name: String,
-                                  configure: CrossProject => CrossProject): MultiScalaCrossProject =
-    new MultiScalaCrossProject(platforms, name, configure)
-}
-
-class MultiScalaCrossProject(platforms: Seq[Platform],
-                             name: String,
-                             configure: CrossProject => CrossProject)
+class MultiScalaCrossProject(name: String,
+                             configureCommonJvm: CrossProject => CrossProject,
+                             configureJs: CrossProject => CrossProject,
+                             configureNative: CrossProject => CrossProject)
     extends MultiScala {
-
-  def apply(scalaV: String): CrossProject = apply(scalaV, scalaV, x => x)
-
   def apply(
-      scalaV: String,
-      scalaVJs: String,
-      configurePerScala: CrossProject => CrossProject = x => x
+      platforms: Seq[Platform],
+      scalaV: String
   ): CrossProject = {
+    val hasJs     = platforms.contains(JSPlatform)
+    val hasNative = platforms.contains(NativePlatform)
     val projectId = projectIdPerScala(name, scalaV)
-    val resultingProject =
+    val res =
       CrossProject(
         id = projectId,
         base = file(s".cross/$projectId")
       )(platforms: _*)
         .crossType(CrossType.Full)
         .withoutSuffixFor(JVMPlatform)
         .settings(moduleName := name)
-        .jvmSettings(scalaVersion := scalaV)
-        .jsSettings(scalaVersion := scalaVJs)
-        .nativeSettings(scalaVersion := scalaV)
+        .settings(scalaVersion := scalaV)
         .settings(srcFull(name))
 
-    configurePerScala(configure(resultingProject))
+    val conf = configureCommonJvm
+      .andThen(if (hasJs) configureJs else identity)
+      .andThen(if (hasNative) configureNative else identity)
+    conf(res)
   }
 }