main/postmarketos-ui-gnome: don't mark UI as hardware accelerated

While GNOME does require hardware acceleration, GNOME's X11 mode (not sure about Wayland mode) does work without DRM support, so it works fine with software rendering on devices without a working GPU as long as fbdev works.

I think this raises the question of whether we should rename pmb:hw-accel to pmb:drm as e.g. Phosh should work without 3D acceleration if e.g. a device uses a kernel with SimpleDRM support, but that is out of scope for this MR/commit.

Effectively reverts 638362de