Fix function `remove_fragment()` in wsrep_schema so that no error is
raised if the fragment to be removed is not found in the
wsrep_streaming_log table. This is necessary to handle the case where
streaming transaction in idle state is BF aborted. This may result in
the case where the rollbacker thread successfully removes the
transaction's fragments, followed by the applier's attempt to remove
the same fragments. Causing the node to leave the cluster after
reporting a "Failed to apply write set" error.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>