text: Title: Add support for removing a registered Client
  Body: ## Problem
  
  <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
  
  While it is not standard practice to create multiple Prisma Client's in production code, it could be very useful to be able to "unregister" a Client instance in unit tests. We already provide a context manager solution in the form of:
  
  ```py
  from prisma.testing import reset_client
  
  with reset_client():
      ...
  ```
  
  However a context manager is not always an elegant solution.
  
  ## Suggested solution
  
  <!-- A clear and concise description of what you want to happen. -->
  
  Add a corresponding `deregister` function.