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

idea/idea-completion/testData/basic/multifile/ExcludedJavaClass/somePackage/ExctraClass.java

@@ -1,6 +1,6 @@
 package somePackage;
 
-// Such strange name here needed to make completion prefix not intersecting with kotlin.ExtensionFunctionType
+// Such strange name here needed to make completion prefix not intersecting with kotlin.Extension
 public class ExctraClass {
 
-}
+}