lz4: Remove ancient test helpers

Building lz4 as root was causing `make clean` to fail with permission
errors.

We used to have to install lz4 from source back in Ubuntu 14.04, but
nowadays the installed lz4 is fine. Get rid of ancient helpers and
cruft!

.github/workflows/dev-short-tests.yml

@@ -170,7 +170,6 @@ jobs:
     - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # tag=v4.2.2
     - name: LZ4, thread pool, and libs build testslib wrapper test
       run: |
-        make lz4install
         make -C tests test-lz4
         make check < /dev/null | tee   # mess with lz4 console detection
         make clean