gdb, remote: adjust debug printing

remote::wait () may get called rather frequently, polluting the logging
output with tons of

    [remote] wait: enter
    [remote] wait: exit

messages.

Similarly, remote_target::remote_notif_remove_queued_reply () will print
the debug message even if nothing was actually removed.  Change that to
only print a debug message if a stop reply was removed.

Approved-By: Tom Tromey <tom@tromey.com>