Remove conflicting AsRef implementation for PyObject

Problem

A compilation conflict exists in vm/src/object/core.rs where there's a custom implementation of AsRef<Self> for PyObject that conflicts with the standard library's blanket implementation impl<T> AsRef<T> for T.

This creates a coherence conflict and will cause compilation failures.

Location

File: vm/src/object/core.rs around lines 865-870

Context

This issue was identified during the review of PR #5892 which aims to use Self where possible throughout the codebase.

Backlinks

• Related PR: Use Self where possible #5892
• Comment: Use Self where possible #5892 (comment)

Requested by: @youknowone