Remove TypeSystemContext::mayBeTypeVariable optimization

It doesn't work now since anyway it's required to obtain type
constructor
At the same time, now it's not very expensive to obtain the constructor
since it's just a lookup tag

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/components/ClassicTypeSystemContextForCS.kt

@@ -34,11 +34,6 @@ class ClassicTypeSystemContextForCS(override val builtIns: KotlinBuiltIns) : Typ
         return this.freshTypeConstructor
     }
 
-    override fun KotlinTypeMarker.mayBeTypeVariable(): Boolean {
-        require(this is KotlinType, this::errorMessage)
-        return this.constructor is TypeVariableTypeConstructor
-    }
-
     override fun createCapturedType(
         constructorProjection: TypeArgumentMarker,
         constructorSupertypes: List<KotlinTypeMarker>,

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/components/ConstraintInjector.kt

@@ -219,7 +219,7 @@ class ConstraintInjector(
 
         // from AbstractTypeCheckerContextForConstraintSystem
         override fun isMyTypeVariable(type: SimpleTypeMarker): Boolean =
-            type.mayBeTypeVariable() && c.allTypeVariables.containsKey(type.typeConstructor())
+            c.allTypeVariables.containsKey(type.typeConstructor())
 
         override fun addUpperConstraint(typeVariable: TypeConstructorMarker, superType: KotlinTypeMarker) =
             addConstraint(typeVariable, superType, UPPER)