[FIR] Remove the redundant isFunctionType check

^KT-63865 fixed

compiler/fir/analysis-tests/testData/resolveWithStdlib/castForFunctionsWithDifferentArities.kt

@@ -0,0 +1,9 @@
+// ISSUE: KT-63865
+
+fun test(b: Boolean, block1: Any.() -> Unit, block2: (Any.(Any?) -> Unit)?) {
+    if (b) {
+        requireNotNull(block1)
+    } else {
+        requireNotNull(block2)
+    }
+}