[FIR] Remove the redundant isFunctionType check

^KT-63865 fixed

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

@@ -0,0 +1,12 @@
+FILE: castForFunctionsWithDifferentArities.kt
+    public final fun test(b: R|kotlin/Boolean|, block1: R|kotlin/Any.() -> kotlin/Unit|, block2: R|(kotlin/Any.(kotlin/Any?) -> kotlin/Unit)?|): R|kotlin/Unit| {
+        when () {
+            R|<local>/b| ->  {
+                R|kotlin/requireNotNull|<R|kotlin/Any.() -> kotlin/Unit|>(R|<local>/block1|)
+            }
+            else ->  {
+                R|kotlin/requireNotNull|<R|kotlin/Any.(kotlin/Any?) -> kotlin/Unit|>(R|<local>/block2|)
+            }
+        }
+
+    }

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)
+    }
+}