Issue #1360 - Part 1: Simplify layers acceleration prefs.

This gets rid of platform-dependent hard-coded defaults, but keeps
build-time blocking if there is no GL provider (in which case layers
acceleration almost certainly won't work because it needs a GL
compositor and would likely crash without)

New prefs are
- layers.acceleration.enabled to enable HWA
- layers.acceleration.force to force it enabled (requires .enabled to be
  set as well)

This is the platform part of this issue. The rest will be front-end work
(Preference UI integration and pref migration)

gfx/thebes/gfxPlatformMac.cpp

@@ -372,12 +372,6 @@ gfxPlatformMac::ReadAntiAliasingThreshold()
     return threshold;
 }
 
-bool
-gfxPlatformMac::AccelerateLayersByDefault()
-{
-  return true;
-}
-
 // This is the renderer output callback function, called on the vsync thread
 static CVReturn VsyncCallback(CVDisplayLinkRef aDisplayLink,
                               const CVTimeStamp* aNow,