[FIR] Fix enhancement of FlexibleNullability and EnhancedNullability

- Add utilities to add new attribute to ConeAttributes
- Get rid of FlexibleNullability attribute (it can be easily inferred
    for any flexible type at any moment)
- Fix determining of EnhancedNullability attribute

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/TypeUtils.kt

@@ -200,9 +200,6 @@ fun FirTypeRef.isUnsafeVarianceType(session: FirSession): Boolean {
 fun FirTypeRef.hasEnhancedNullability(): Boolean =
     coneTypeSafe<ConeKotlinType>()?.hasEnhancedNullability == true
 
-fun FirTypeRef.hasFlexibleNullability(): Boolean =
-    coneTypeSafe<ConeKotlinType>()?.hasFlexibleNullability == true
-
 fun FirTypeRef.withoutEnhancedNullability(): FirTypeRef {
     require(this is FirResolvedTypeRef)
     if (!hasEnhancedNullability()) return this