FIR: revert a hack that allows overriding T!! with T

1. this should've been only done if the language feature for validating
    that is disabled;

 2. that feature probably won't matter by the time FIR is stable;

 3. it only worked because type enhancement of type arguments is broken
    anyway - a more correct hack would be to provide a custom
    ConeTypePreparator.

compiler/fir/analysis-tests/testData/resolve/kt41984.fir.txt

@@ -20,7 +20,7 @@ FILE: main.kt
 
     }
     public final fun test_1(b: R|B<kotlin/Int>|, x: R|kotlin/Int|, inv: R|Inv<kotlin/Int>|): R|kotlin/Unit| {
-        R|<local>/b|.R|SubstitutionOverride</B.take: R|kotlin/String|>|(R|<local>/x|)
-        R|<local>/b|.<Inapplicable(INAPPLICABLE): /B.take>#(Null(null))
+        R|<local>/b|.<Ambiguity: take, [/B.take, /B.take]>#(R|<local>/x|)
+        R|<local>/b|.<Ambiguity: take, [/B.take, /B.take]>#(Null(null))
         R|<local>/b|.R|SubstitutionOverride</B.takeInv: R|kotlin/String|>|(R|<local>/inv|)
     }

compiler/fir/analysis-tests/testData/resolve/kt41984.kt

@@ -16,8 +16,8 @@ public abstract class A<T, V> {
 
 class Inv<T>
 
-open class B<V> : A<Any, V>() {
-    override fun take(value: V): String {
+open <!ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED!>class B<!><V> : A<Any, V>() {
+    <!NOTHING_TO_OVERRIDE!>override<!> fun take(value: V): String {
         return ""
     }
 
@@ -25,7 +25,7 @@ open class B<V> : A<Any, V>() {
 }
 
 fun test_1(b: B<Int>, x: Int, inv: Inv<Int>) {
-    b.take(x)
-    b.take(<!NULL_FOR_NONNULL_TYPE!>null<!>)
+    b.<!OVERLOAD_RESOLUTION_AMBIGUITY!>take<!>(x)
+    b.<!NONE_APPLICABLE!>take<!>(null)
     b.takeInv(inv)
 }