outdated install instructions for rail_flexzboost in the docs under "Algorithm / architecture specific issues" section

[sschmidt23]

I was just looking at the docs page, and noticed that there is a very outdated instruction for installing flexzboost that recommends that you install xgboost==0.90.0. Advising people to install this older version of xgboost is no longer necessary thanks to Biprateep tracking down a few FlexCode issues, so we should definitely remove that from the readthedocs.

[sschmidt23]

Actually, it looks like the rail_bpz section is also out of date. Given the RAIL packages instructions that are now above on the readthedocs installation page, I think both the flexzboost and BPZ_lite sections under "Algorithm/architecture specific issues" can just be deleted.