mirrors.bbclass: remove redundant server-specific mirrors

With the previously added general git repo fallback rule the server
specific fallback mirrors for git.savannah.gnu.org and
git.yoctoproject.org are redundant. Remove them.

Signed-off-by: Daniel Wagenknecht <dwagenknecht@emlix.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>