[FIR] KT-57195: Don't approximate ft when checking override return types

Why does this check even exist? K1 doesn't do it.

^KT-57195 Fixed

compiler/testData/diagnostics/tests/overrideNotNull_Fail.kt

@@ -0,0 +1,12 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57195
+
+import java.io.File
+
+open class AbstractFE1UastTest {
+    open var testDataDir = File("").parentFile
+}
+
+class Legacy: AbstractFE1UastTest() {
+    override var testDataDir: File = File("").parentFile // K1: ok, K2: VAR_TYPE_MISMATCH_ON_OVERRIDE
+}