Stop forcing Wayland for Qt

[ilya-fedin]

tdesktop has recently been updated to core20 that has Qt 5.12 and, therefore, has support for native Wayland.
Qt desktop helper has a block that forces Wayland even if a compositor is blacklisted by Qt

GNOME is blacklisted due to a bug that fixed in upstream a long time ago, but still actual in 18.04 LTS. But, since snapcraft-desktop-helpers forces Wayland, people see this flickering by default, what is not a good UX and complains in tdesktop issues.

I believe that the fix for mutter should be backported to 18.04 LTS or snapcraft-desktop-helpers should stop forcing Wayland.