su: remove program (util-linux is now the best source for it)

su: remove program (util-linux is now the best source for it)

* README: Omit "su" from list of programs.
* src/su.c: Remove file.
* src/Makefile.am: Remove su-related rules and variables.
* tests/misc/su-fail: Remove test.
* tests/Makefile.am (TESTS): Remove misc/su-fail.
* tests/misc/invalid-opt: Remove su-related code.
* src/.gitignore: Remove su.
* man/su.x: Remove file.
* man/Makefile.am (su.1): Remove rule.
* po/POTFILES.in: Remove su.c from the list.
* TODO: Remove ancient entry.
* NEWS (Changes in behavior): Mention it.
* doc/coreutils.texi: Remove su-related description.
* AUTHORS: Remove su.
* m4/lib-check.m4 (cu_LIB_CHECK): Remove file/macro.
* configure.ac: Remove su-related code and sole use of cu_LIB_CHECK.
* scripts/git-hooks/commit-msg: Remove su from this list, too.