[identity] Remove google_auth_provider from SDK.

From a conversation with Alain, it seems the consumers requesting the
inclusion of the auth provider in the SDK have now moved off sdk_image
to another more specific product configuration. Google_auth_provider
is now pretty useless without account_manager, and since I need to
rename it while moving it out of Topaz I'd really like to just remove it
from the SDK.

AUTH-164 #comment

Change-Id: Ic0f2f03918169e0e82acf46063cbbba27e839af0