"Redundant Unit" inspection: new tests, simplification & fixes

Related to KT-23977 and KT-24066

idea/testData/inspectionsLocal/redundantUnitExpression/labeledReturnAnyWithParameters.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+fun foo(f: (Unit, Int, String) -> Any) {}
+
+fun test() {
+    foo { _, _, _ ->
+        return@foo Unit<caret>
+    }
+}

idea/testData/inspectionsLocal/redundantUnitExpression/returnAsNullableAny.kt

@@ -0,0 +1,5 @@
+// PROBLEM: none
+
+fun f(): Any? {
+    return Unit<caret>
+}