remove GCM Mock API in favor of integration tests

[aabmass]

Need to update code introduced in #83 when these issues are closed:
googleinterns/cloud-operations-api-mock#58
googleinterns/cloud-operations-api-mock#56

[aabmass]

The cloud mock project has been archived and we will be using integration tests against real gcm in place of this. I think the fix here is to migrate to integration tests and remove the cloud mock dep altogether.

[aabmass]

This is done for monitoring, would just need to dome something similar for trace.

[aabmass]

Triaged again, still low priority.