mailmap: remove the "repo-abbrev" comment

Remove the magical "repo-abbrev" comment added when this file was
introduced in e0ab1ec9

 ([PATCH] add .mailmap for proper
git-shortlog output, 2007-02-14).

It's been an undocumented feature of git-shortlog(1), originally added
to git for Linus's use. Since then he's no longer using it[1], and
I've removed the feature in git.git's 4e168333a87 (shortlog: remove
unused(?) "repo-abbrev" feature, 2021-01-12). It's on the "master"
branch, but not yet in a release version.

Let's also remove it from linux.git, both as a heads-up to any
potential users of it in linux.git whose use would be broken sooner
than later by git itself, and because it'll eventually be entirely
redundant.

1. https://lore.kernel.org/git/CAHk-=wixHyBKZVUcxq+NCWMbkrX0xnppb7UCopRWw1+oExYpYw@mail.gmail.com/

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>