Remove support for v1 of the PCRE library

Remove support for v1 of the PCRE library

Remove support for using version 1 of the PCRE library. Its use has
been discouraged by upstream for a long time, and it's in a
bugfix-only state.

Anyone who was relying on v1 in particular got a nudge to move to v2
in e6c531b808 (Makefile: make USE_LIBPCRE=YesPlease mean v2, not v1,
2018-03-11), which was first released as part of v2.18.0.

With this the LIBPCRE2 test prerequisites is redundant to PCRE. But
I'm keeping it for self-documentation purposes, and to avoid conflict
with other in-flight PCRE patches.

I'm also not changing all of our own "pcre2" names to "pcre", i.e. the
inverse of 6d4b5747f0 (grep: change internal *pcre* variable &
function names to be *pcre1*, 2017-05-25). I don't see the point, and
it makes the history/blame harder to read. Maybe if there's ever a
PCRE v3...

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>