Revert "ci: cache/restore opam"

`ocaml/setup-ocaml` might do this for us?

This reverts commit 8749644.

.github/workflows/server-ci.yml

@@ -26,32 +26,15 @@ jobs:
       - name: Check out repository
         uses: actions/checkout@v4
 
-      - name: Restore cached ~/.opam
-        id: cache-opam-restore
-        uses: actions/cache/restore@v4
-        with:
-          path: ~/.opam
-          key: ${{ matrix.ocaml-version }}
-
       - name: Set up OCaml
-        if: steps.cache-opam-restore.outputs.cache-hit != 'true'
         uses: ocaml/setup-ocaml@v2
         with:
           ocaml-compiler: ${{ matrix.ocaml-version }}
 
       - name: Install server dependencies
-        if: steps.cache-opam-restore.outputs.cache-hit != 'true'
         working-directory: ./cn-lsp/server
         run: opam install . --deps-only --locked -y
 
-      - name: Cache ~/.opam
-        if: steps.cache-opam-restore.outputs.cache-hit != 'true'
-        id: cache-opam-save
-        uses: actions/cache/save@v4
-        with:
-          path: ~/.opam
-          key: ${{ steps.cache-opam-restore.outputs.cache-primary-key }}
-
       - name: Build server
         working-directory: ./cn-lsp/server
         run: |