desktop-shell: Remove unnecessary type casts
desktop-shell: Remove unnecessary type casts

Remove a few instances of casting weston_seat to weston_seat.

Signed-off-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>
Reviewed-by: Jon A. Cruz <d4ae605477945a543a3ecda8955c93978db16a84@osg.samsung.com>

kwm81/weston,mchalupa/weston,xorgy/weston,Gnurou/weston,giucam/weston,eyolfson/weston,eyolfson/weston,Fantu/compositor-spice,xorgy/weston,krezovic/weston,giucam/weston,Fantu/compositor-spice,Fantu/compositor-spice,kwm81/weston,Gnurou/weston,mchalupa/weston,sir-murray/weston,jonnylamb/weston,Tarnyko/weston-xdg_surface_present,sir-murray/weston,udoprog/weston,Tarnyko/weston-xdg_surface_present,udoprog/weston,jonnylamb/weston,krezovic/weston