Removed "Supported commands" from header comment.

The inline CLI usage comments in every function already provide sufficient
documentation for usage of each individual functionality, which are nicely
organized and rendered in the generated HTML docs:
http://docs.saltstack.com/en/latest/ref/clouds/all/salt.cloud.clouds.gce.html

Per discussion with module author (@erjohnso), dropping this comment block.
Merge pull request #17137 from mbrukman/clouds-gce-cleanup-comment

Removed "Supported commands" from header comment.