Get rid of native-mt leftovers (Kotlin#4013)

Notably, an artificial entry point

Fixes Kotlin#4009

Author: qwwdfsad

kotlinx-coroutines-core/build.gradle

@@ -108,8 +108,6 @@ kotlin {
     targets.withType(org.jetbrains.kotlin.gradle.plugin.mpp.KotlinNativeTargetWithTests.class).configureEach {
         binaries.getTest("DEBUG").with {
             optimized = true
-            // Test for memory leaks using a special entry point that does not exit but returns from main
-            freeCompilerArgs += ["-e", "kotlinx.coroutines.mainNoExit"]
         }
 
         binaries.test("workerTest", [DEBUG]) {

kotlinx-coroutines-core/native/src/WorkerMain.native.kt

@@ -20,9 +20,3 @@ fun mainBackground(args: Array<String>) {
     error("CFRunLoopRun should never return")
 }
 
-// This is a separate entry point for tests with leak checker
-fun mainNoExit(args: Array<String>) {
-    workerMain { // autoreleasepool to make sure interop objects are properly freed
-        testLauncherEntryPoint(args)
-    }
-}

kotlinx-coroutines-core/nativeDarwin/src/WorkerMain.kt

@@ -16,8 +16,3 @@ fun mainBackground(args: Array<String>) {
         exitProcess(result)
     }.result // block main thread
 }
-
-// This is a separate entry point for tests with leak checker
-fun mainNoExit(args: Array<String>) {
-    testLauncherEntryPoint(args)
-}