Please remove some NativeMML hacks for Firefox

Created by: fred-wang

The following hacks should be removed in Firefox >= 29 but they still seem to be enabled in the develop branch:

nMML.forceReflow nMML.widthBug MML.mtdWidthBug

https://github.com/mathjax/MathJax/blob/develop/unpacked/jax/output/NativeMML/jax.js#L1310