Remove rpz->db_registered

Remove rpz->db_registered

As pointed out in !813 db_registered is sort of redundant.  It is
set to `true` only in `dns_zone_rpz_enable_db()` right before the
`dns_rpz_dbupdate_callback()` callback is registered.  It is only
required in that callback and it is the only place that the callback
is registered.  Therefore there is no path that that `REQUIRE` can
fail.

The `db_registered` variable is only set to `false` in
`dns_rpz_new_zone`, so it is not like the variable is unset again
later.

The only other place where `db_registered` is checked is in
`rpz_detach()`.  If `true`, it will call
`dns_db_updatenotify_unregister()`.  However if that happens, the
`db_registered` is not set back to `false` thus this implies that
this may happen multiple times.  If called a second time, most
likely the unregister function will return `ISC_R_NOTFOUND`, but
the return value is not checked anyway.  So it can do without the
`db_registered` check.