Drop some unimplemented methods in GitRef (#1525)

There are two unimplemented methods in GitRef, which are mostly covered
by Commit.get_statuses(), remove them.

github/GitRef.py

@@ -92,22 +92,6 @@ class GitRef(github.GithubObject.CompletableGithubObject):
         )
         self._useAttributes(data)
 
-    def get_statuses(self):
-        """
-        https://developer.github.com/v3/repos/statuses/#list-statuses-for-a-specific-ref
-        :calls: `GET /repos/:owner/:repo/commits/:ref/statuses`
-        :return:
-        """
-        pass
-
-    def get_status(self):
-        """
-        https://developer.github.com/v3/repos/statuses/#get-the-combined-status-for-a-specific-ref
-        :calls: `GET /repos/:owner/:repo/commits/:ref/status`
-        :return:
-        """
-        pass
-
     def _initAttributes(self):
         self._object = github.GithubObject.NotSet
         self._ref = github.GithubObject.NotSet

github/GitRef.pyi

@@ -9,8 +9,6 @@ class GitRef(CompletableGithubObject):
     def _useAttributes(self, attributes: Dict[str, Any]) -> None: ...
     def delete(self) -> None: ...
     def edit(self, sha: str, force: Union[bool, _NotSetType] = ...) -> None: ...
-    def get_statuses(self) -> None: ...
-    def get_status(self) -> None: ...
     @property
     def object(self) -> GitObject: ...
     @property