Remove client package from repository (#153)

The protobuf definitions and the client should be maintained in separate
repositories for better modularity and maintainability.

This PR removes the client from this repository, ensuring that only the
protobuf specifications remain here. The client will be migrated to
[frequenz-client-weather-python](https://github.com/frequenz-floss/frequenz-client-weather-python).

Author: TalweSingh

RELEASE_NOTES.md

@@ -15,7 +15,8 @@
   horizon of returned forecasts. When not specified, forecasts with their
   minimum/maximum available horizon will be returned.
 
-* Add a function to the client to upsample forecast data.
+* ~~Add a function to the client to upsample forecast data.~~ The client will be moved to
+  [frequenz-client-weather-python](https://github.com/frequenz-floss/frequenz-client-weather-python).
 
 <!-- Here goes the main new features and examples or instructions on how to use them -->