Remove in-tree flatpak (#4856)

.github/workflows/flatpak.yml

@@ -1,109 +0,0 @@
-name: Flatpak
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
-
-on:
-  push:
-    branches:
-      - "develop"
-      - "release-*"
-    # We don't do anything with these artifacts on releases. They go to Flathub
-    tags-ignore:
-      - "*"
-    paths:
-      # File types
-      - "**.cpp"
-      - "**.h"
-      - "**.java"
-      - "**.ui"
-
-      # Build files
-      - "flatpak/**"
-
-      # Directories
-      - "buildconfig/**"
-      - "cmake/**"
-      - "launcher/**"
-      - "libraries/**"
-      - "program_info/**"
-      - "tests/**"
-
-      # Files
-      - "CMakeLists.txt"
-      - "COPYING.md"
-
-      # Workflows
-      - ".github/workflows/flatpak.yml"
-  pull_request:
-    paths:
-      # File types
-      - "**.cpp"
-      - "**.h"
-      - "**.java"
-      - "**.ui"
-
-      # Build files
-      - "flatpak/**"
-
-      # Directories
-      - "buildconfig/**"
-      - "cmake/**"
-      - "launcher/**"
-      - "libraries/**"
-      - "program_info/**"
-      - "tests/**"
-
-      # Files
-      - "CMakeLists.txt"
-      - "COPYING.md"
-
-      # Workflows
-      - ".github/workflows/flatpak.yml"
-  workflow_dispatch:
-
-permissions:
-  contents: read
-
-jobs:
-  build:
-    name: Build (${{ matrix.arch }})
-
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - os: ubuntu-22.04
-            arch: x86_64
-
-    runs-on: ${{ matrix.os }}
-
-    container:
-      image: ghcr.io/flathub-infra/flatpak-github-actions:kde-6.10
-      options: --privileged
-      volumes:
-        - /opt/hostedtoolcache:/hostedtoolcache
-
-    steps:
-      - name: Free up disk space
-        shell: bash
-        run: |
-          rm -rf /hostedtoolcache/*
-
-      - name: Checkout repository
-        uses: actions/checkout@v6
-        with:
-          submodules: true
-
-      - name: Set short version
-        shell: bash
-        run: |
-          echo "VERSION=${GITHUB_SHA::7}" >> "$GITHUB_ENV"
-
-      - name: Build Flatpak
-        uses: flatpak/flatpak-github-actions/flatpak-builder@v6
-        with:
-          bundle: PrismLauncher-${{ runner.os }}-${{ env.VERSION }}-Flatpak.flatpak
-          manifest-path: flatpak/org.prismlauncher.PrismLauncher.yml
-          arch: ${{ matrix.arch }}