pcre: Drop internal libpcre copy

It’s no longer used and is a maintenance burden.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #962

docs/reference/glib/meson.build

@@ -25,8 +25,6 @@ if get_option('gtk_doc')
     'gtrace-private.h',
     'glib-mirroring-tab',
     'gnulib',
-    'pcre',
-    'update-pcre',
     'gbytesprivate.h',
     'gvariant-internal.h',
     'gvariant-serialiser.h',