astropy.modeling.NonLinearLeastSquares does not support bounds

scipy's nnls doesn't support bounds, and at present I don't think astropy's does either, so this just removes support for bounds in nnls.

@nden - you should check this and let me know if I'm mistaken. 

I'm interested to see any other bounded solvers implemented.  Note that scipy may eventually get the BVLS solver:
https://github.com/scipy/scipy/issues/3130
so it looks like there will be more bounded fitters available in the nearish future.