flatpak: remove references to systemd unit

Replaces #7676

When building as a flatpak, don't install the systemd user services
since flatpaks can't use them. Remove references to the systemd service
from the DBus service.

Also, customize the app metadata depending on the debug mode.

Co-authored-by: Leorize <leorize+oss@disroot.org>

flatpak/com.mitchellh.ghostty-debug.yml

@@ -6,11 +6,7 @@ sdk-extensions:
   - org.freedesktop.Sdk.Extension.ziglang
 default-branch: tip
 command: ghostty
-# Integrate the rename into zig build, maybe?
-rename-desktop-file: com.mitchellh.ghostty.desktop
-rename-appdata-file: com.mitchellh.ghostty.metainfo.xml
 rename-icon: com.mitchellh.ghostty
-desktop-file-name-suffix: " (Debug)"
 finish-args:
   # 3D rendering
   - --device=dri