SystemTools: Fix Touch to avoid requiring file ownership

SystemTools::Touch should only require +w file permissions. The previous
implementation used `stat` to get the current time, and then passed that
time to one of utimensat/utimes/utime. However, utimes/utimensat only
permit an arbitrary-time argument to be passed by the file owner.

Therefore, per the docs, we pass NULL for the `times` argument to utimes
and utimensat, to indicate using the current time. This is permitted for
any UID with +w on the file.

Remove plain `utime` fallback that is no longer necessary.