Fix warning: assigned but unused variable

Fix warning: assigned but unused variable

There are several places where varaibles are assigned but not used.
The variables can be replaced with _, or prepended with _ to avoid a warning.
In one case the variable was removed because it was at the end of the method.

Addresses the following warnings:

lib/google/api_client.rb:493: warning: assigned but unused variable - key
lib/google/api_client/batch.rb:168: warning: assigned but unused variable - callback
lib/google/api_client/batch.rb:227: warning: assigned but unused variable - base
lib/google/api_client/batch.rb:271: warning: assigned but unused variable - protocol
lib/google/api_client/batch.rb:271: warning: assigned but unused variable - reason
lib/google/api_client/request.rb:247: warning: assigned but unused variable - request_env