utime: remove obsolete module

utime: remove obsolete module

This module, like autoconf's AC_FUNC_UTIME_NULL macro, has been
unnecessary for years, and has been marked as obsolete for 10 months.
* modules/utime: Remove file.
* lib/utime.c: Remove file.
* m4/utime.m4: Remove file.
* m4/utimes-null.m4: Remove file.
* doc/posix-functions/utime.texi (utime): Remove reference to
the module.  Move the sole "fixed by gnulib" item into the
"problems not fixed by Gnulib" list.
* MODULES.html.sh (func_all_modules): Remove reference to "utime".