[FIR] Don't enhance Int and String Java final static fields

It's effectively a breaking change (^KT-62558)

K2 assigns flexible type to all static fields (from Java) and, for example,
`String? becomes String unlike K1. It affects IR signature generating.
That's why signature dump is disabled for some tests.

^KT-57811 Fixed
^KT-61786 Fixed

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/enhancement/SignatureEnhancement.kt

@@ -117,14 +117,7 @@ class FirSignatureEnhancement(
                 val newReturnTypeRef = enhanceReturnType(
                     firElement, emptyList(), firElement.computeDefaultQualifiers(),
                     predefinedEnhancementInfo = null
-                ).let {
-                    val lowerBound = it.type.lowerBoundIfFlexible()
-                    if ((lowerBound.isString || lowerBound.isInt) && firElement.isStatic && firElement.initializer != null) {
-                        it.withReplacedConeType(it.type.withNullability(ConeNullability.NOT_NULL, session.typeContext))
-                    } else {
-                        it
-                    }
-                }
+                )
 
                 val symbol = FirFieldSymbol(original.callableId)
                 buildJavaField {