monitor: Drop dead QMP check from monitor_read_password()

Function is only called in HMP context since commit 333a96e "qapi:
Convert change".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
