As a side-effect of purge_relay_logs(), sql_slave_skip_counter
was silently ignored in GTID mode.

But sql_slave_skip_counter in fact is not a good match with GTID.
And it is not really needed either, as users can explicitly set
@@gtid_slave_pos to skip specific GTIDs, in a way that matches
well how GTID replication works.

So with this patch, we give an error on attempts to set
sql_slave_skip_counter when using GTID, with a suggestion to use
gtid_slave_pos instead, if needed.