Revert r348438.

The fix to override the default python version when building
the sysutils/py-google-compute-engine did not work, and there
are still issues that need to be addressed in the port itself.

See bugzilla 238267 for additional details.

MFC after: 6 days
MFC with: r348438
MFC note: no-op to appease the merge tracker
Sponsored by: The FreeBSD Foundation