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|)
     }