Remove `SemanticModel#find_binding` (#6546)
## Summary
This method is almost never what you actually want, because it doesn't
respect Python's scoping semantics. For example, if you call this within
a class method, it will return class attributes, whereas Python actually
_skips_ symbols in classes unless the load occurs within the class
itself. I also want to move away from these kinds of dynamic lookups and
more towards `resolve_name`, which performs a lookup based on the stored
`BindingId` at the time of symbol resolution, and will make it much
easier for us to separate model building from linting in the near
future.
## Test Plan
`cargo test`
