Bug 971943: 1. Remove unused variable. r=me.
Bug 971943: 2. webgl.force-layers-readback preference moved to gfxPrefs.  r=gw280
Bug 971943: 3. gfx.apitrace.enabled (ANDROID only) preference moved to gfxPrefs.  r=gwright
Bug 971943: 4. gfx.layerscope.port and gfx.layerscope.enabled moved to gfxPrefs. r=dglastonbury