bus_config_parser_check_doctype: Remove, unused

bus_config_parser_check_doctype: Remove, unused

We have never checked the <!DOCTYPE> of busconfig XML since the libxml
parser was removed in 2013, and the libxml parser was broken before
that anyway. The recommended Expat parser (our only parser since 2013)
does not appear to have ever validated this, so now does not seem like
the time to start. Just ignore the <!DOCTYPE> if there is one.

(We never validated this particularly strictly anyway;
<!DOCTYPE busconfig SYSTEM "http://example.com/bees"> would have been
treated as perfectly valid.)

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=107739