rust: remove dead utils module
This was introduced a while back while not being imported anywhere in the
module system. How pertinent this Facebook module was to the codebase was
debatable at the time, nowadays with the Rust VFS layer having laid its
foundations, it is clear that we should drop it.
Differential Revision: https://phab.mercurial-scm.org/D10890
#require rustfmt test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
$ for f in `testrepohg files 'glob:**/*.rs'` ; do
> $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
> done