[ty] Optimize and simplify some object-related code (#22366)

## Summary

I wondered if this might improve performance a little. It doesn't seem
to, but it's a net reduction in LOC and I think the changes make sense.
I think it's worth it anyway just in terms of simplifying the code.

## Test Plan

Our existing tests all pass and the primer report is clean (aside from
our usual flakes).

crates/ty_python_semantic/resources/mdtest/import/builtins.md

@@ -32,6 +32,7 @@ typeshed = "/typeshed"
 `/typeshed/stdlib/builtins.pyi`:
 
 ```pyi
+class object: ...
 class Custom: ...
 
 custom_builtin: Custom