ci(emacs-28): disable nightly builds from emacs-28 branch

Emacs 28.x does no longer receive daily work, as next is 29.x being
worked on in emacs-29.

.github/workflows/nightly-emacs-28.yml

@@ -1,46 +0,0 @@
----
-name: Nightly (emacs-28)
-on:
-  schedule:
-    - cron: "0 23 * * *"
-  workflow_dispatch:
-    inputs:
-      git_sha:
-        description: Override Emacs git commit SHA to build
-        required: false
-
-jobs:
-  prepare:
-    name: Prepare
-    uses: ./.github/workflows/_prepare.yml
-    secrets:
-      TAP_REPO_TOKEN: ${{ secrets.TAP_REPO_TOKEN }}
-
-  build:
-    name: Build
-    needs: [prepare]
-    uses: ./.github/workflows/_build.yml
-    with:
-      os: macos-11
-      git_ref: emacs-28
-      git_sha: ${{ github.event.inputs.git_sha }}
-      build_args: --native-comp
-    secrets:
-      APPLE_DEVELOPER_CERTIFICATE_P12_BASE64: ${{ secrets.APPLE_DEVELOPER_CERTIFICATE_P12_BASE64 }}
-      APPLE_DEVELOPER_CERTIFICATE_PASSWORD: ${{ secrets.APPLE_DEVELOPER_CERTIFICATE_PASSWORD }}
-      KEYCHAIN_PASSWORD: ${{ secrets.KEYCHAIN_PASSWORD }}
-      AC_USERNAME: ${{ secrets.AC_USERNAME }}
-      AC_PASSWORD: ${{ secrets.AC_PASSWORD }}
-      AC_PROVIDER: ${{ secrets.AC_PROVIDER }}
-      AC_SIGN_IDENTITY: ${{ secrets.AC_SIGN_IDENTITY }}
-      TAP_REPO_TOKEN: ${{ secrets.TAP_REPO_TOKEN }}
-
-  release:
-    name: Release
-    needs: [build]
-    uses: ./.github/workflows/_release.yml
-    with:
-      plan_artifact: build-plan
-      dmg_artifact: dmg
-    secrets:
-      TAP_REPO_TOKEN: ${{ secrets.TAP_REPO_TOKEN }}