Cleanup variables in wayland.gyp

enable_ozone_wayland_vkb and enable_xdg_shell where used to enable or
disable support for xdg_shell and Wayland VKB. We do a run time check
for xdg_shell support already, this CL makes similar change for VKB
and gets rid of these variables.
Merge branch 'master' of https://github.com/01org/ozone-wayland