gdb: remove an out of date comment about disp_del_at_next_stop

gdb: remove an out of date comment about disp_del_at_next_stop

Delete an out of date comment about disp_del_at_next_stop, the comment
says:

  /* NOTE drow/2003-09-08: This state only exists for removing
     watchpoints.  It's not clear that it's necessary...  */

I'm sure this was true when the comment was added, but today the
disp_del_at_next_stop state is not just used for deleting watchpoints,
which leaves us with "It's not clear that it's necessary...", which
doesn't really help at all.

And then this comment is located on one random place where
disp_del_at_next_stop is used, rather than at its definition site.

Lets just delete the comment.

No user visible changes after this commit.

Reviewed-By: Pedro Alves <pedro@palves.net>