Revert "don't tempalte register"

This reverts commit 7ba790bbaf.

Fixes #15700

lib/ansible/playbook/task.py

@@ -245,13 +245,6 @@ class Task(Base, Conditional, Taggable, Become):
 
         super(Task, self).post_validate(templar)
 
-    def _post_validate_register(self, attr, value, templar):
-        '''
-        Override post validation for the register args field, which is not
-        supposed to be templated
-        '''
-        return value
-
     def _post_validate_loop_args(self, attr, value, templar):
         '''
         Override post validation for the loop args field, which is templated