Removed unused context in _extend_snapshot method

Input parameter 'context' of _extend_snapshot method in
cinder/api/extended_snapshot_attributes.py is not being used, so remove
this 'context' parameter.

Change-Id: I8577e3938d5ed5bfb143a605320063a6b17c3be0
Closes-bug: #1279146
Merge "Removed unused context in _extend_snapshot method"