Fix tests: "infix modifier required" and "operator modifier required" errors

compiler/testData/codegen/box/regressions/kt3173.kt

@@ -1,4 +0,0 @@
-fun box(): String {
-    val sum = fun Int.(other: Int) = this + other
-    return if (1 sum 2 == 3) "OK" else "Fail"
-}

compiler/testData/codegen/box/regressions/kt3421.kt

@@ -1,5 +1,5 @@
 public object Globals{
-    public fun get(key: String, remove: Boolean = true): String {
+    operator fun get(key: String, remove: Boolean = true): String {
         return "OK"
     }
 }

compiler/testData/codegen/box/regressions/kt3999.kt

@@ -1,17 +0,0 @@
-fun test() {
-  val a: (@Extension Function2<*, *, *>).(@Extension Function2<*, *, *>)->Unit = {}
-  val b: (@Extension Function2<*, *, *>).(@Extension Function2<*, *, *>)->Unit = {"$this $it"}
-  val c: (@Extension Function2<*, *, *>).(@Extension Function2<*, *, *>)->Unit = {}
-  a.b(c)
-  a b c
-}
-
-fun Int.foo(a: Int) = this * a
-val boo = fun Int.(a: Int): Int = this + a
-
-fun box(): String {
-    test()
-    1 foo 2
-    3 boo 4
-    return "OK"
-}