Tests: remove diagnostic test with incorrect compiler behavior

In fact the latest compiler (neither K1 nor K2) does NOT report an error
here, see KT-66522. The error was there in the diagnostic test because
the test used custom code which invoked parts of the old JVM backend to
report signature clash errors.

The issue is rather minor and is present since 1.5, so to reduce
confusion, the test is deleted.

 #KT-66522

compiler/testData/diagnostics/testsWithStdLib/RedeclarationMainInMultiFileClass.fir.kt

@@ -1,11 +0,0 @@
-// FILE: a.kt
-@file:JvmName("Util")
-@file:JvmMultifileClass
-
-fun main() {}
-
-// FILE: b.kt
-@file:JvmName("Util")
-@file:JvmMultifileClass
-
-fun main() {}

compiler/testData/diagnostics/testsWithStdLib/RedeclarationMainInMultiFileClass.kt

@@ -1,11 +0,0 @@
-// FILE: a.kt
-@file:JvmName("Util")
-@file:JvmMultifileClass
-
-<!CONFLICTING_JVM_DECLARATIONS, CONFLICTING_JVM_DECLARATIONS!>fun main()<!> {}
-
-// FILE: b.kt
-@file:JvmName("Util")
-@file:JvmMultifileClass
-
-<!CONFLICTING_JVM_DECLARATIONS, CONFLICTING_JVM_DECLARATIONS!>fun main()<!> {}

compiler/testData/diagnostics/testsWithStdLib/RedeclarationMainInMultiFileClass.txt

@@ -1,4 +0,0 @@
-package
-
-public fun main(): kotlin.Unit
-public fun main(): kotlin.Unit