e_comp_wl: Remove unnecessary subsurface check

e_comp_wl: Remove unnecessary subsurface check

The current code includes a check if the ec is for a subsurface to
prevent compositor from considering wayland_tbm buffer transform,
assuming that clients should not set the wayland_tbm buffer transform
for subsurfaces.

However, this assumption is implicit and makes the code harder to
maintain. To maintain consistency with other surfaces, it is better to
remove the subsurface check.

If a client attempts to set the wayland_tbm buffer transform for a
subsurface, it should be handled in different way - It's undefined
behavior for now. For example, we can raise a protocol error to be
explicit.

This change removes the unnecessary subsurface check, making the code
more maintainable and consistent with other surfaces.

Change-Id: I036ee6cf162edc65f953b64ffce5c8268871dfa2