Make X·M·L types take priority over plaintext ones

If `XMLTYPES` defines something as X·M·L, it should be treated as
X·M·L, regardless of whether there is a parser which claims to support
it.

This avoids awkward footguns where a parser might transform and claim
support for (through `@书社:supported-media-types`) an X·M·L dialect,
resulting in ⛩️📰 书社 treating that type as plaintext and wrapping it
in an `<html:script>` element. X·M·L types listed in
`@书社:supported-media-types` should instead not have any effect
(⛩️📰 书社 should not require parsers to reparse X·M·L).

This commit also removes the unused `simpletypes` variable; it was
formerly used for categorization of types into plaintext or asset prior
to the implementation of automatic detection.