doc-diff: drop SOURCE_DATE_EPOCH override

doc-diff: drop SOURCE_DATE_EPOCH override

The original doc-diff script set SOURCE_DATE_EPOCH to make asciidoc's
output deterministic. Otherwise, the mtime of the source files would end
up in the footer of the manpage, causing noisy and uninteresting diff
hunks.

But this has been unused since 28fde3a1f4 (doc: set actual revdate for
manpages, 2023-04-13), as the footer uses the externally-specified
GIT_DATE instead (that needs to be set consistently, too, which it now
is as of the previous commit).

Asciidoc sets several automatic attributes based on the mtime (or manual
epoch), so it's still possible to write a document that would need
SOURCE_DATE_EPOCH set to be deterministic. But if we wrote such a thing,
it's probably a mistake, and we're better off having doc-diff loudly
show it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>