mdate-sh: ignore $SOURCE_DATE_EPOCH (again).

* lib/mdate-sh: pay no attention to SOURCE_DATE_EPOCH.
The previous change (2025-05-15) was a misunderstanding;
looking at SOURCE_DATE_EPOCH causes spurious makeinfo rebuilds:
https://lists.gnu.org/archive/html/automake/2025-06/msg00021.html
* t/mdate5.sh: remove check for this.
* t/txinfo-vtexi4.sh: likewise.
* doc/automake.texi (Texinfo): remove mention of this.
* NEWS: update.
5 files changed