Remove oauth2client

[whilp]

This library is deprecated:

https://google-auth.readthedocs.io/en/latest/oauth2client-deprecation.html

Its presence complicates use of containerregistry in Python3. Fortunately, this is the only import:

containerregistry/client/docker_creds_.py

Line 32 in 2f1cfd3

from oauth2client import client as oauth2client

And I can't even find a use of that import in docker_creds_. The only other reference in that file is a comment:

containerregistry/client/docker_creds_.py

Lines 116 to 122 in 2f1cfd3

	# WORKAROUND...
	# The python oauth2client library only loads the credential from an
	# on-disk cache the first time 'refresh()' is called, and doesn't
	# actually 'Force a refresh of access_token' as advertised.
	# This call will load the credential, and the call below will refresh
	# it as needed. If the credential is unexpired, the call below will
	# simply return a cache of this refresh.

Given that oauth2client doesn't seem to actually be used for anything and that removal would reduce the list of deps, simplify Python3 support, shall we?