Remove flake8

[oskooi]

The Python command-line utility flake8 for ensuring code consistency is currently not doing anything as there is no pre-commit hook for it. It only runs if someone remembers to invoke it manually. On top of that, Black already handles the formatting/style rules (E/W codes). The .flake8 config ignores most codes that would overlap (E501, W503, E203).

What's left that Black doesn't cover:

• F-codes (pyflakes): unused imports, undefined names, redefined variables. These catch real bugs - this is the only genuinely useful part.
• C-codes (complexity): set ot 57 (which is extremely high; relaxed projects use 15-20), so effectively disabled.
• B-codes (bugbear): selected in the config, but flake8-bugbear isn't installed, so these do nothing.

.flake8 can be removed without meaningful impact to readability. Black already owns formatting. The one important thing we lose is pyflakes-style error detection (unused imports, undefined names), but since flake8 isn't wired into pre-commit, we are not getting that benefit today.