fix buggy assert in plugin_extfile
Remove SPF RR-type

These were never widely useful in practice thanks to the
ambiguity in the original standard.  As of RFC 7208,
Type-99 SPF records are not recommended for use at all anymore.

Refs:
http://tools.ietf.org/html/rfc6686#section-6
http://tools.ietf.org/html/rfc7208#section-3.1
http://tools.ietf.org/html/rfc7208#section-14.1