window: Relax restrictions for can_tile_maximized ()

The current code requires windows to be resizable to be considered
for tiling, which excludes all maximized/tiled windows. While this
restriction concurs with the desired behavior for edge-tiling, it
feels overly restrictive for keybindings.
As the edge-tiling code in update_move() already ensures the above
restriction, it seems save to remove it from the can_tile_maximized()
function, assuming that windows that are not meant to be tiled or
maximized won't provide a maximize function.

https://bugzilla.gnome.org/show_bug.cgi?id=648700