`magit-log-create-author-date-overlays': remove redundant check

Commit d1e19af8 merged `magit-log-create-author-date-overlay'
into `magit-log-set-author-date-overlays' (renamed to
`magit-log-create-author-date-overlays'), but didn't remove the
second check of `magit-log-author-date-overlay'.  This variable
isn't modified between the first and second check, so the latter
is redundant.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #902 from praet/author-date-overlays

`magit-log-create-author-date-overlays': remove redundant check