[CI] Remove symlink workaround

Now we have Spack build cache in `/opt/spack`, so that we shouldn't hit the
GitHub Action issue.

.github/workflows/UnitTests.yml

@@ -502,13 +502,6 @@ jobs:
     - name: Set MPITRAMPOLINE_MPIEXEC
       run: echo "MPITRAMPOLINE_MPIEXEC=$(which mpiexec)" >> "${GITHUB_ENV}"
 
-    # Inside containers, GitHub Actions remaps `/home/runner/work` to `/__w`, but this
-    # results into a broken path. See <https://github.com/actions/runner/issues/2185>.
-    # Let's create this now missing symlink to work around this shenanigan.
-    - name: Create symlink
-      run: |
-        ln -s /home/runner/work/github-actions-buildcache /__w/github-actions-buildcache
-
     - name: Build MPIwrapper
       run: |
         apt-get update