IR text tests: Don't dump signatures for local declarations

1. Local declarations don't participate in IR-linkage, because they
can be referenced only inside the same body -> can be dropped
from IR text tests.

2. Mangled names for private declarations computed by descriptors/fir
are actually not used anywhere (they are recomputed by IR
immediately before serialization of IR). But sometimes such
mangled names diverge between K1 and K2 -> don't check them, but
always check mangled names computed by IR even for private
declarations.

3. Also: Drop DUMP_LOCAL_DECLARATION_SIGNATURES test directive.

^KT-57428 Obsolete
^KT-57430 Obsolete
^KT-57434 Obsolete
^KT-57778 Obsolete
^KT-57775 Obsolete

compiler/testData/ir/irText/firProblems/AssignmentOperator.sig.kt.txt

@@ -93,8 +93,10 @@ fun <T : Any?> id(t: T): T
 //   Mangled name: foo#main(){}
 //   Mangled name for the signature computed from Ir:         main(){}
 //   Mangled name for the signature computed from Descriptor: main(){}%AssignmentOperator.kt
-//   Public signature: foo/main|7392120756922895329[0]
-//   Public signature debug description: main(){}%AssignmentOperator.kt
+//   Public signature by IR: foo/main|-4284757841571462650[0]
+//   Public signature by IR debug description: main(){}
+//   Public signature by Frontend: foo/main|7392120756922895329[0]
+//   Public signature by Frontend debug description: main(){}%AssignmentOperator.kt
 // CHECK JS_IR NATIVE:
 //   Mangled name: foo#main(){}
 //   Public signature: foo/main|-4284757841571462650[0]