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/localClasses.sig.kt.txt

@@ -3,19 +3,4 @@
 //   Public signature: /outer|1607449560170809066[0]
 //   Public signature debug description: outer(){}
 fun outer(): Unit
-  // CHECK:
-  //   Mangled name: #outer(){}.LocalClass
-  //   Mangled name for the signature: outer(){}
-  local class LocalClass {
-    // CHECK:
-    //   Mangled name: #outer(){}.LocalClass#<init>(){}
-    //   Mangled name for the signature: outer(){}<init>(){}
-    constructor() /* primary */
-
-    // CHECK:
-    //   Mangled name: #outer(){}.LocalClass#foo(){}
-    //   Mangled name for the signature: outer(){}foo(){}
-    fun foo(): Unit
-
-  }