Jeremy deleted rootlessAccelInit's implementation in 2008 in commit
587c010a. Delete its prototype and the
remaining commented-out call to it.

It still makes sense for the rootless GC ops to relax the planemask, but
that's independent of the size of the operation, so quit checking the
thresholds there.

FillBytes and CompositePixels are not called anywhere, so delete
everything related to both.

Signed-off-by: Jamey Sharp <jamey@minilop.net>
Cc: Jeremy Huddleston <jeremyhu@apple.com>
Cc: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>