Remove --galera-sst-mode option from mysqldump

mysqldump's galera-sst-mode was introduced to help transfer the donor node's gtid
binlog state to the joiner node. wsrep_sst_mysqldump script invokes mysqldump
with this additional option to make mysql dump some additional commands to
turn off binary logging and setting gtid_binlog_state variable.

However, these additional commands can be implemented in the wsrep_sst_mysqldump
script itself and thus the option can be removed from mysqldump.