composite: Remove a misleading comment

This comment is above compIsAlternateVisual, which used to be the only
thing determining whether implicit redirect was needed. It's not anymore
due to the redirection exception list. That job is now performed by
compImplicitRedirect, whose code is self-explanitory.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>