libweston: drop a misleading dmabuf comment

This comment was added in 230f3b1b with the intent that if we had an information table about pixel formats (which we do have today), we could implement more sanity checks like ensuring that width pixels fit into stride.

Daniel Vetter said on #dri-devel IRC recently:

< danvet> since userspace shouldn't look at stride for buffers with
modifiers, only pass it around unchanged

I asked for clarification. It was expected that userspace would not do any kind of sanity checks as modifiers could change everything, even the DRM core did not do the kind of checks we already have in Weston.

Let's remove the misleading code comment so that people don't get the idea of adding more well-intended but ill-advised sanity checks. If more checks are added, they must take the modifier into account, which the existing checks do not do.

After 5 years, it is far too late to remove our existing sanity checks, but we can attempt to not cause any more damage.