meson: generate HTML pages for all man page categories

meson: generate HTML pages for all man page categories

When generating HTML pages for our man pages we only generate them for
category 1 in Meson, which are the pages corresponding to our built-in
commands. I cannot tell why I added this filter though: our Makefile
installs all man pages, so a Meson-based build misses out on many of
them.

Fix this by removing the filter.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>