Early models of the C-10 (1000-GTR/Concours) from 1986-1993 did have a fork oil drain screw. From 1994 on, there was none.
On late model C-10's, you can drain the fork oil out several ways. Turn the bike upside down over a bucket, or remove the fork cap (jack the front end up first!) and suction it out, or drain it during a normal/regular fork seal replacement.
Rick