Replace "flatMap -> flatten" inspection with abstract "simplifiable call"

Related to KT-30501

idea/testData/inspectionsLocal/collections/simplifiableCall/notOnlyReference.kt

@@ -0,0 +1,5 @@
+// PROBLEM: none
+// WITH_RUNTIME
+fun test() {
+    listOf(listOf(1)).flatMap<caret> { it + 1 }
+}