[FIR2IR] Unconditionally disable linkViaSignatures on JVM

It was an experimental compilation mode that we don't need anymore.

KT-64809

compiler/testData/ir/irText/declarations/mainFunctionManglingInJvm.kt

@@ -4,6 +4,9 @@
 // FIR_IDENTICAL
 // WITH_STDLIB
 
+// SEPARATE_SIGNATURE_DUMP_FOR_K2
+// ^ The main function has a different mangled name when it's computed from its K1 descriptor in K/JVM.
+
 // FILE: a.kt
 fun main() {
     println("main() in a.kt")