Doc: remove src/backend/regex/re_syntax.n.

We aren't publishing this file as documentation, and it's been
much more haphazardly maintained than the real docs in func.sgml,
so let's just drop it.  I think the only reason I included it in
commit 7bcc6d98 was that the Berkeley-era sources had had a man
page in this directory.

Discussion: https://postgr.es/m/4099447.1614186542@sss.pgh.pa.us