Remove legacyBehavior from Link component on Next.js 15

[fuma-nama]

Goals

Next.js 15 as a major release, it's a good time to remove the legacy behavior from Next.js Link component.
It was added in Next.js 13, after two major releases, I think it makes sense to remove it.

This can:

• Reduce the effort to maintain the component
• Reduce the confusion for developers when they saw these props which are no longer documented in the App Router docs.

Non-Goals

Add new features

Background

When playing with Next.js 15 canary, I found some props like as are no longer documented while they are kept in the implementation.

I was surprised by the complexity of the implementation of the Next.js Link component, some legacy functionalities should be removed.

Proposal

• Remove legacyBehavior prop
• Remove as prop
• Remove passHref prop

I'm very willing to provide a PR