Revert "Rename Extension annotation to ExtensionFunctionType"

This reverts commit 460dad135c.

The old compiler will not see the new runtime correctly. Postponing this until
1.0

compiler/testData/diagnostics/tests/inline/binaryExpressions/arrayAccess.kt

@@ -12,7 +12,7 @@ operator fun <T, U, V> Function2<T, U, V>.get(index : Int) {
 
 }
 
-operator fun <T, U, V, W> @ExtensionFunctionType Function3<T, U, V, W>.get(index : Int) {
+operator fun <T, U, V, W> @Extension Function3<T, U, V, W>.get(index : Int) {
 
 }