Remove `B904`'s lowercase exemption (#5751)
## Summary
It looks like bugbear, [from the
start](https://github.com/PyCQA/flake8-bugbear/pull/181#issuecomment-904314876),
has had an exemption here to exempt `raise lower_case_var`. I looked at
Hypothesis and Trio, which are mentioned in that issue, and Hypothesis
has exactly one case of this, and Trio has none, so IMO it doesn't seem
worth special-casing.
Closes https://github.com/astral-sh/ruff/issues/5664.
