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/expressions/implicitNotNullInDestructuringAssignment.sig.kt.txt

@@ -6,8 +6,7 @@ operator fun J?.component1(): Int
 
 // CHECK JVM_IR:
 //   Mangled name: #component2@J(){}kotlin.Int
-//   Mangled name for the signature: component2@J(){}kotlin.Int
-//   Public signature: [ File 'implicitNotNullInDestructuringAssignment.kt' <- /component2|-623550756830644944[0] ]
+//   Public signature: [ File '/implicitNotNullInDestructuringAssignment.kt' <- /component2|-623550756830644944[0] ]
 private operator fun J.component2(): Int
 
 // CHECK: