Remove KotlinCompilerVersion.IS_PRE_RELEASE

Now, milestones of major Kotlin releases (e.g. 1.6.0-M1) will NOT
generate prerelease binaries anymore.

The reason for that is it's proven to be quite a complicated process to
turn on IR_PRE_RELEASE after the release is branched, perform double
bootstrap and fix tests, and then do it again in reverse just before
release. With the new release cadence, we don't have that much time to
do it and verify that everything works as intended.

Note that this only removes the "global" prerelease flag. Compiler will
still generate prerelease binaries if a non-stable language version is
used. For example, Kotlin 1.6.0-M1 with `-language-version 1.7` will
generate prerelease binaries.

compiler/testData/compileKotlinAgainstCustomBinaries/preReleaseCompilerAgainstPreReleaseLibraryLatestStable/library/a.kt

@@ -1,5 +0,0 @@
-package a
-
-class A
-
-fun A.foo() = ""

compiler/testData/compileKotlinAgainstCustomBinaries/preReleaseCompilerAgainstPreReleaseLibraryLatestStable/output.txt

@@ -1 +0,0 @@
-OK

compiler/testData/compileKotlinAgainstCustomBinaries/preReleaseCompilerAgainstPreReleaseLibraryLatestStable/source.kt

@@ -1,7 +0,0 @@
-package usage
-
-import a.*
-
-fun baz(param: A) {
-    param.foo()
-}

compiler/testData/compileKotlinAgainstCustomBinaries/preReleaseCompilerAgainstPreReleaseLibraryStableLanguageVersion/library/a.kt

@@ -1,5 +0,0 @@
-package a
-
-class A
-
-fun foo() = ""

compiler/testData/compileKotlinAgainstCustomBinaries/preReleaseCompilerAgainstPreReleaseLibraryStableLanguageVersion/output.txt

@@ -1 +0,0 @@
-OK

compiler/testData/compileKotlinAgainstCustomBinaries/preReleaseCompilerAgainstPreReleaseLibraryStableLanguageVersion/source.kt

@@ -1,8 +0,0 @@
-package usage
-
-import a.*
-
-fun baz(param: A): A {
-    foo()
-    return param
-}