remove original callback call since it already is in the queue

this commit fixes the strange behaviour of callbacks being called twice
for one cached callback / request.

the callback is added to the cache (QueueableCacheStorage) for the GET
request. the onResponseReceived() method calls the original callback AND
all callbacks found for the request. the original callback is already in
the cache, so it is being called twice.

people involved:
Dennis Schumann (https://github.com/dschumann, dennis.schumann@devbliss.com)
Alexander Bluem (https://github.com/jopek, alexander.bluem@devbliss.com)