regex: remove obsolete macro AM_WITH_REGEX

regex: remove obsolete macro AM_WITH_REGEX

Today, practically nobody uses the GNU rx library, which, according
to its own website <http://www.gnu.org/software/rx/rx.html>, has
been "decommissioned".  Consequently, the automake-provided macro
AM_WITH_REGEX is not used nor required anymore.

See also commits `v1.11-587-g5f335be' and `v1.11-433-g37b0aee',
where that macro had been deprecated.

* m4/regex.m4: Delete.
* m4/Makefile.am (dist_automake_ac_DATA): Do not list it anymore.
* doc/automake.texi (Obsolete Macros): Remove description, and in
fact any mention, of `AM_WITH_REGEX'.
* tests/regex.test: Delete.
* tests/help-regex.test: Likewise.
* tests/regex-obsolete.test: Likewise.
* tests/Makefile.am (TESTS): Do not list them anymore.
* NEWS: Update.