Remove the OpRetryError exception

This is only used in two places, in an error path that is no longer
valid since Ganeti 2.0. We remove the try..except since we should not
get it anymore (and if we do, then we should catch it in all
config.Update cases) and we remove the exception class completely.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>