Remove obsolete (to UWP) build scripts and update build configs

The following files have been removed:
- `cron_publish_flatpak.yml`: Automated Flathub release publishing.
- `linux_build_flatpak.yml`: Steps for building and publishing Flatpak.
- `linux_build_qt.yml`: Steps for building Linux app using Qt.
- `appimage-qt.sh`: Script for creating AppImage for PCSX2.
- `build-dependencies-qt.sh`: Script for building various dependencies.
- `20-sdl2.json`: Build manifest for SDL2.
- `net.pcsx2.PCSX2.json`: Flatpak manifest for PCSX2.
- `pcsx2-qt.metainfo.xml.in`: Metadata for PCSX2 application.
- `build-dependencies.sh`: Script for setting up and building dependencies.