Remove support for building Alpha/PR (adhoc) builds.

We've haven't really been using these and they're proving to be quite a chore to keep running.
Also, we recently needed an external demo build and we chose to go with custom build on top of testflight instead.

As such it's been decided that we can go ahead and remove them.

.github/workflows/pr-build.yml

@@ -1,65 +0,0 @@
-name: PR Build
-
-on:
-  pull_request:
-    types: [ labeled, synchronize, opened, reopened ]
-
-jobs:
-  build:
-    # Only run for PRs that contain the trigger label. The action will fail for forks due to
-    # missing secrets, but there's no need to handle this as it won't run automatically.
-    if: contains(github.event.pull_request.labels.*.name, 'Trigger-PR-Build')
-
-    name: Release
-    runs-on: macos-15
-    
-    concurrency:
-      # Only allow a single run of this workflow on each branch, automatically cancelling older runs.
-      group: alpha-${{ github.head_ref }}
-      cancel-in-progress: true
-
-    steps:
-      - uses: actions/checkout@v4
-
-      - uses: actions/cache@v4
-        with:
-          path: vendor/bundle
-          key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-gems-
-
-      - name: Setup environment
-        env:
-          CI_WORKFLOW: PR_BUILD
-        run:
-          source ci_scripts/ci_common.sh && setup_github_actions_environment
-
-      - name: Import signing certificate
-        uses: apple-actions/import-codesign-certs@cfd6eb39a2c848ead8836bda6b56813585404ba7 # v5
-        with:
-          p12-file-base64: ${{ secrets.ALPHA_CERTIFICATES_P12 }}
-          p12-password: ${{ secrets.ALPHA_CERTIFICATES_P12_PASSWORD }}
-
-      - name: Build alpha
-        run: bundle exec fastlane build_alpha
-        env:
-          APPSTORECONNECT_KEY_ID: ${{ secrets.APPSTORECONNECT_KEY_ID }}
-          APPSTORECONNECT_KEY_ISSUER_ID: ${{ secrets.APPSTORECONNECT_KEY_ISSUER_ID }}
-          APPSTORECONNECT_KEY_CONTENT: ${{ secrets.APPSTORECONNECT_KEY_CONTENT }}
-          DIAWI_API_TOKEN: ${{ secrets.DIAWI_API_TOKEN }}
-          GITHUB_PR_NUMBER: ${{ github.event.number }}
-          BROWSERSTACK_USERNAME: ${{ secrets.BROWSERSTACK_USERNAME }}
-          BROWSERSTACK_ACCESS_KEY: ${{ secrets.BROWSERSTACK_ACCESS_KEY }}
-
-      - name: Add release notes and Diawi info
-        uses: NejcZdovc/comment-pr@a423635d183a8259308e80593c96fecf31539c26 #v2.1.0
-        with:
-          message: |
-            :iphone: Scan the QR code below to install the build for this PR. 
-            :lock: This build is for internal testing purpose. Only devices listed in the ad-hoc provisioning profile can install Element Alpha.  
-
-            ![QR code](${{ env.DIAWI_QR_CODE_LINK }})  
-
-            If you can't scan the QR code you can install the build via this link: ${{ env.DIAWI_FILE_LINK }}
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}