[ci] Drop GitHub Action seanmiddleditch/gha-setup-ninja (#6044)

From the GitHub Action README: "This action is no longer
necessary, as ninja is now included on all default
GitHub runner instances."

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2149685..0889ec6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1191,9 +1191,6 @@
       - name: Update CMake
         uses: jwlawson/actions-setup-cmake@v2.2
 
-      - name: Install ninja-build tool
-        uses: seanmiddleditch/gha-setup-ninja@v6
-
       - name: Run pip installs
         run: |
           python -m pip install --upgrade pip