OPT_MAKE_FLAGS() used to emit two options (one with "no" prefixed),
but that has been long removed by special casing flag options in the
option parser. OPT_FLAG_ON() used to imply that there's no "no-"
prefixed option, but this hasn't been the case for a while either.
(Conceptually, it has been replaced by OPT_FLAG_STORE().)
Remove OPT_FLAG_OFF, which was unused.