flatpak: Drop --socket=x11
The documentation states that: The fallback-x11 option makes the X11 socket available only if there is no Wayland socket. This option was introduced in 0.11.3. To support older Flatpak releases, specify both x11 and fallback-x11. The fallback-x11 option takes precedence when both are supported.
But given that 0.11.3 was released over three years ago, it seems safe to drop it and rely on fallback-x11 only.