Remove -Xmax-classfile-name; hard-code to 240, like Scala 2.13.0-RC1

[IDoCodingStuffs]

scala/scala#7497 for reference

EDIT: About #5812, doesn't quite fix it.

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[Blaisorblade]

Both on latest release and this PR, tests/pending/run/t8199.scala still fails to compile by creating files with name of ~260 chars. So I'd merge the PR but keep the issue open since the underlying feature apparently doesn't work (and it's not this PR's fault).

sbt:dotty> dotc tests/pending/run/t8199.scala
[warn] Multiple main classes detected.  Run 'show discoveredMainClasses' to see the list
[info] Running (fork) dotty.tools.dotc.Main -classpath /Users/pgiarrusso/.ivy2/cache/org.scala-lang/scala-library/jars/scala-library-2.12.8.jar:/Users/pgiarrusso/git/dotty/library/../out/bootstrap/dotty-library-bootstrapped/scala-0.13/dotty-library_0.13-0.13.0-bin-SNAPSHOT.jar tests/pending/run/t8199.scala
java.nio.file.FileSystemException: ./reallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongname$cls01234567890.class: File name too long
	at sun.nio.fs.UnixException.translateToIOException(UnixException.java:91)
	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
	at sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:214)
	at java.nio.file.Files.newByteChannel(Files.java:361)
	at java.nio.file.Files.createFile(Files.java:632)
	at dotty.tools.io.AbstractFile.fileOrSubdirectoryNamed(AbstractFile.scala:245)
	at dotty.tools.io.AbstractFile.fileNamed(AbstractFile.scala:256)
	at scala.tools.nsc.backend.jvm.BytecodeWriters.getFile(BytecodeWriters.scala:35)
	at scala.tools.nsc.backend.jvm.BytecodeWriters.getFile$(BytecodeWriters.scala:28)
	at dotty.tools.backend.jvm.GenBCodePipeline.getFile(GenBCode.scala:73)
	at scala.tools.nsc.backend.jvm.BCodeHelpers.getFileForClassfile(BCodeHelpers.scala:38)
	at scala.tools.nsc.backend.jvm.BCodeHelpers.getFileForClassfile$(BCodeHelpers.scala:37)
	at dotty.tools.backend.jvm.GenBCodePipeline.getFileForClassfile(GenBCode.scala:73)
	at dotty.tools.backend.jvm.GenBCodePipeline$Worker1.$anonfun$visit$3(GenBCode.scala:256)
	at scala.collection.immutable.List.map(List.scala:290)

$ echo 'reallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongnamereallylongname$cls01234567890.class'|wc
       1       1     260

[Blaisorblade]

This PR does its job correctly; technical debt gets in its way.

[Blaisorblade]

I'm happy with this code — but letting @smarter pick the best action.

[IDoCodingStuffs]

Would we want a separate issue for the test failing to compile? Scope of the addressed issue seems to be limited to matching 2.13

[IDoCodingStuffs]

@smarter Is there anything else that needs to be taken care of or is this good for a merge?