subs: Don't squash telegrams together when time goes backwards

subs: Don't squash telegrams together when time goes backwards

We've always squashed them when the time difference is smaller than
TEL_SECONDS, regardless of sign.  This involves passing the difference
to abs(), implicitly casting from time_t to int, which triggers a
Clang warning.

I could clean this up to get rid of the warning, but time should never
go backwards, and trying to make things prettier when it does isn't
worthwhile.  Simply drop the abs().

While there, drop the function comment.  It's been inaccurate since
Empire 3 dropped mail.c, and bogus since commit 17223e8 (v4.3.29)
added tel_cont.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>