Fix(core): Remove deprecated experiments.lazyBarrel config for RsPack

[VedikaGupt]

… 1.7+

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Rspack 1.6+ enables experiments.lazyBarrel by default, making the explicit configuration unnecessary.
Starting with Rspack 1.7, setting this option triggers a deprecation warning, and it will be removed entirely in Rspack 2.0.
This PR removes the deprecated configuration to avoid warnings and keep the build aligned with current and upcoming Rspack behavior.

Changes

Removed the deprecated experiments.lazyBarrel configuration from packages/docusaurus/src/webpack/base.ts
Removed the DISABLE_RSPACK_LAZY_BARREL environment variable check, which is no longer needed now that the feature is stable and enabled by default

Test Plan

Ran build locally
Verified no behavioral change in the build output
Lazy barrel optimization continues to work as expected via Rspack defaults

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

Fixes #11688

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[VedikaGupt]

@slorber @Josh-Cena Hey, can you please let me know if any other changes are required to address this issue or can we check this in? Happy to contribute