From a user comment on code.blender.org:

"i tried the test build a few days back to find that with the “new” fbx 7.3 exporter you can’t select “deform bones only” which means if you try using the rigify or makehuman rig it will export all the bones in the file.

would it be possible to add it back into the options, btw its still there for the 6.1 version

tested on UE4.1"