Remove pyproject.toml from fixtures directory (#8726)

## Summary

This exists to power a test, but it ends up affecting the behavior of
all files in the directory. Namely, it means that these files _aren't_
excluded when you format or lint them directly, since in that case, Ruff
will fall back to looking at the `pyproject.toml` in
`crates/ruff_linter/resources/test/fixtures`, which _doesn't_ exclude
these files, unlike our top-level `pyproject.toml`.

crates/ruff_linter/resources/test/fixtures/README.md

@@ -1,3 +0,0 @@
-# fixtures
-
-Fixture files used for snapshot testing.

crates/ruff_linter/resources/test/fixtures/pyproject.toml

@@ -1,10 +0,0 @@
-[tool.ruff]
-line-length = 88
-extend-exclude = [
-  "excluded_file.py",
-  "migrations",
-  "with_excluded_file/other_excluded_file.py",
-]
-
-[tool.ruff.lint]
-per-file-ignores = { "__init__.py" = ["F401"] }