Remove github/codespaces-precache action

This action was deprecated on 2022-04-22.  Its repo has since been
deleted, causing the build to fail:

https://github.com/rails/rails/runs/6162755138?check_suite_focus=true#step:1:31

Note that the current way to configure prebuilds appears to be through
GitHub's web UI:

https://docs.github.com/en/codespaces/prebuilding-your-codespaces/configuring-prebuilds

.github/workflows/codespaces.yml

@@ -1,16 +0,0 @@
-name: prebuild codespaces
-on:
-  push:
-    branches:
-      - main
-jobs:
-  create_prebuild:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v3
-      - uses: github/codespaces-precache@v1.0.1
-        with:
-          regions: "WestUs2 EastUs WestEurope SouthEastAsia"
-          sku_name: largePremiumLinux
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}