text: Title: Delete/remove client method (flask client)
  Body: I can only speak for the Flask client as that is what I'm using. It would be great if the `oauth.register(...)` method had an `oauth.remove(client_name)`.
  
  **Is your feature request related to a problem? Please describe.**
  I wanted to write a test for something and it involved registering an oauth2 (flask) client for testing purposes. After the tests complete, I want a clean way to remove that testing client from the oauth object, but I cannot see a way to do this. 
  
  Without it, running the test poses the following issues. (1) If I create the client in my test with a fixed name (e.g. 'Test Client'), it will return the first client from the oauth registry for all subsequent tests, instead of creating a new client each time. But if I create the client with a unique name for each test (e.g. 'Test Client 1', 'Test Cleint 2', ...), the `oauth` registry will grow for as many clients are registered until the server restarts. Obviously the latter is the expected behaviour, but a `.remove_client` method seems missing.
  
  **Describe the solution you'd like**
  Just like with `oauth.register(...)`, maybe something like `oauth.remove_client(client_name)` would be nice. For example, in the [https://github.com/lepture/authlib/blob/master/authlib/integrations/base_client/registry.py](https://github.com/lepture/authlib/blob/master/authlib/integrations/base_client/registry.py), we could add a new method:
  
  ```python
  def remove(self, client_name):
      if client_name not in self._registry:
          raise KeyError(f"Client {client_name} not found in registry.")
  
      if client_name not in self._clients:
          raise KeyError(f"Client {client_name} not found in registry.")
  
      del self._registry[client_name]
      del self._clients[client_name]
  ```
  
  But I am not sure if this is sufficient or if it overlooks other issues.
  
  **Describe alternatives you've considered**
  As far as I can tell, we might be able to do this by simply editing the internal `oauth._clients` and `oauth._registry` dictionaries. For example `del oauth._clients[my_test_client_name]; del oauth._registry[my_test_client_name]` but I don't know if this fully accomplishes the goal. It definitely doesn't feel nice. 
  