simplemerge: remove now-unused arguments
The `ui` argument is no longer needed now that we don't handle
`--print` in the library code.
Differential Revision: https://phab.mercurial-scm.org/D12151
#require rustfmt test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ RUSTFMT=$(rustup which --toolchain nightly-2021-11-02 rustfmt)
$ for f in `testrepohg files 'glob:**/*.rs'` ; do
> $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
> done