[research proj] [lxmert] remove bleach dependency
github reports `bleach==3.1.5` to have a vulnerability and it's not really used anywhere in the code, and because it has a fixed version set that is vulnerable, so just as well remove it completely from deps.
https://github.com/huggingface/transformers/security/dependabot/examples/research_projects/lxmert/requirements.txt/bleach/open

@LysandreJik, @sgugger, @patrickvonplaten 