Don't auto-link SHA1s and Gerrit change IDs

This reverts commit b1c3632609174f334df99b1be0f876e5a5400c06.
Regexp was wrong, and it is better to do this in Lua anyhow, and thus
preserve the portability of the message for non-wikitext media.

Marked as 1.7.5.

Change-Id: Ia6d1519c802b3ef00746decb60320c93daff4a18