syntax-check: Simplify VC_LIST_ALWAYS_EXCLUDE_REGEX

syntax-check: Simplify VC_LIST_ALWAYS_EXCLUDE_REGEX

Most of the pattern is no longer relevant, because the files it
was intended to match have been dropped from the repository.

Specifically:

               files        commit        date
  ------------------  ------------  ----------
               *.gif  6cb131e5cbd0  2022-01-19
               *.fig  9ad637c9651f  2020-07-10
  docs/news*.html.in  f45735786a3d  2020-06-02
        docs/*.patch  6be034a8c062  2018-08-23

We can also avoid having a fallback value for the pattern: that
made sense when the implementation was coming from gnulib, as
they wouldn't be able to know in advance if the user would need
to provide their own exclude patterns, but that scenario is no
longer relevant to us.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>