config: remove now-unused support for "includepaths"
This effectively undoes 081b08e4ea13 (templater: look for mapfiles in
template paths, 2015-05-15).
Differential Revision: https://phab.mercurial-scm.org/D8793
#require rustfmt test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ RUSTFMT=$(rustup which --toolchain nightly rustfmt)
$ for f in `testrepohg files 'glob:**/*.rs'` ; do
> $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
> done