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/classes/delegatedImplementationWithExplicitOverride.sig.kt.txt

@@ -8,11 +8,6 @@ class C : IFooBar {
   //   Public signature debug description: <init>(){}
   constructor() /* primary */
   // CHECK:
-  //   Mangled name: C.$$delegate_0
-  //   Public signature: /C.$$delegate_0|-7286425919675154353[0]
-  //   Public signature debug description: 
-  private /* final field */ val $$delegate_0: FooBarImpl
-  // CHECK:
   //   Mangled name: C#foo(){}
   //   Public signature: /C.foo|-1041209573719867811[0]
   //   Public signature debug description: foo(){}