[ci] Remove automatic version bump PR (taichi-dev#6617)

Fixes taichi-dev#6437


### Brief Summary

Our oncall release process changed and this should be done manually.

.github/workflows/release.yml

@@ -377,20 +377,3 @@ jobs:
         with:
           body: ${{ steps.changelog.outputs.content }}
           tag_name: ${{ github.event.inputs.version }}
-
-      - name: Bump version
-        run: |
-          version_parts=(${RELEASE_VERSION//./ })
-          version_parts[2]=$(expr ${version_parts[2]} + 1)
-          next_version=$(IFS=.; echo "${version_parts[*]}")
-          # Update version.txt
-          git checkout -b "bump/$next_version"
-          echo "$next_version" > version.txt
-          git add version.txt
-          # Commit and push changes
-          git commit -m "Bump version to $next_version"
-          git push origin "bump/$next_version"
-          # Create pull request
-          gh pr create -B master -t "[misc] Bump version to $next_version" -b "Bump version to $next_version"
-        env:
-          GITHUB_TOKEN: ${{ secrets.GARDENER_PAT }}