README: drop note about a cairo build option

That build option has been long gone. cairo-image is the only flavor used
nowadays.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

README.md

@@ -307,8 +307,6 @@ There are still many more details to be decided.
 For packagers
 -------------
 
-Always build Weston with --with-cairo=image.
-
 The Weston project is (will be) intended to be split into several
 binary packages, each with its own dependencies. The maximal split
 would be roughly like this: