255 -> 1.0 for color::rgba + remove dead code lint

In build_display_list_abs (block.rs) and handle_reflow (layout_task.rs)
255 is incorrectly used instead of 1.0 for color::rgba (255 is correct
with color::rgb but *not* color::rgba).

In cors.rs remove the dead code warning for fn clear.
The function shouldn't be removed since it'll be used in the future
(fn clear is an implementation of http://fetch.spec.whatwg.org/#concept-cache-clear).
Merge pull request #2941 from bjwbell/change-255-to-1-for-rgba

255 -> 1.0 for color::rgba + remove dead code lint