Revert "Use Zeus to run integration tests on CI (#212)"

This reverts commit 9e46160.

Unfortunately, Zeus is just not reliable enough to run on CI. It's not
even reliable locally. Sometimes tests will hang for no reason, and
sometimes Zeus will fail with errors like `read unix ->: recvmsg:
message too long`. And unfortunately, once Zeus fails, the best way to
fix it is to restart it. This isn't possible to do in CI tests, and
adding some kind of "with Zeus, then without Zeus" strategy to tests
would add complexity I don't want. I'll have to find a better way.

.github/workflows/super_diff.yml

@@ -103,16 +103,5 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
-      - name: Install Zeus
-        run: gem install zeus
-      - name: Start Zeus
-        uses: JarvusInnovations/background-action@v1
-        with:
-          run: zeus start
-          wait-on: |
-            socket:.zeus.sock
-            file:.zeus.sock
-          wait-for: 15s
-          log-output-if: failure
       - name: Run tests
         run: bundle exec rake --trace