Fix unused variable removal for keyword arguments

[Sanjays2402]

Fixes #372.

An assignment such as user = User.objects.get(id=id) was left unchanged because the removal logic counted every = character as an assignment. Parse the statement to recognize a single name target, then preserve the original expression when removing the unused binding. Chained assignments, unpacking, attribute/subscript assignments, and multiline statements remain unchanged.

With --remove-rhs-for-unused-variables, insert a temporary pass so removing the only statement leaves valid Python; the existing cleanup removes that placeholder when another statement remains.

Validation: all 182 tests in test_autoflake.py passed, including 11 subtests; new regressions failed against unchanged main. The pinned Ty type check, Ruff checks/formatting and the project's fuzz script on both changed Python files passed. Prepared with Codex.