[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.fir.sig.kt.txt

@@ -0,0 +1,41 @@
+// FILE: a.kt
+
+// CHECK JVM_IR:
+//   Mangled name: #main(){}
+//   Mangled name for the signature by IR:       main(){}
+//   Mangled name for the signature by Frontend: main(){}%a.kt
+//   Public signature: /main|-4284757841571462650[0]
+//   Public signature debug description: main(){}
+fun main(): Unit
+
+// FILE: b.kt
+
+// CHECK JVM_IR:
+//   Mangled name: #main(){}
+//   Mangled name for the signature by IR:       main(){}
+//   Mangled name for the signature by Frontend: main(){}%b.kt
+//   Public signature: /main|-4284757841571462650[0]
+//   Public signature debug description: main(){}
+fun main(): Unit
+
+// FILE: c.kt
+package foo
+
+// CHECK JVM_IR:
+//   Mangled name: foo#main(){}
+//   Mangled name for the signature by IR:       main(){}
+//   Mangled name for the signature by Frontend: main(){}%c.kt
+//   Public signature: foo/main|-4284757841571462650[0]
+//   Public signature debug description: main(){}
+fun main(): Unit
+
+// FILE: d.kt
+package foo
+
+// CHECK JVM_IR:
+//   Mangled name: foo#main(){}
+//   Mangled name for the signature by IR:       main(){}
+//   Mangled name for the signature by Frontend: main(){}%d.kt
+//   Public signature: foo/main|-4284757841571462650[0]
+//   Public signature debug description: main(){}
+fun main(): Unit

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")