[FIR] Handle 'EnhancedNullability' more properly

This commit includes three changes:
1. 'EnhancedNullability' is no more set for declaration types
2. It is no more used for conversion types in translator
3. Translator inserts implicit not-null cast only when enhanced type is cast to not-null type.

compiler/fir/analysis-tests/testData/resolveWithStdlib/j+k/BasicWithAnnotatedJava.txt

@@ -5,8 +5,8 @@ FILE: jvm.kt
         }
 
         public final fun test(): R|kotlin/Unit| {
-            lval x: R|@EnhancedNullability kotlin/String| = this@R|/User|.R|/Annotated.foo|(String(123))
-            lval y: R|@EnhancedNullability kotlin/String| = this@R|/User|.R|/Annotated.foo|(Null(null))
+            lval x: R|kotlin/String| = this@R|/User|.R|/Annotated.foo|(String(123))
+            lval y: R|kotlin/String| = this@R|/User|.R|/Annotated.foo|(Null(null))
         }
 
     }