Remove recommendation to always use const for performance optimization

[jesskuras]

Description of what this PR is changing or adding, and why:

Removes the recommendation on the Flutter performance best practices page (/perf/best-practices) to always use const constructors on widgets as much as possible for performance optimization, along with the unused flutter_lints reference links and parenthetical mention of const constructors.

Issues fixed by this PR (if any):

N/A

PRs or commits this PR depends on (if any):

None

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[gemini-code-assist]

Code Review

This pull request updates the performance best practices documentation in best-practices.md by removing the recommendation to use const constructors on widgets, as well as the references to the flutter_lints package and its migration guide. There are no review comments, so I have no feedback to provide.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit de33608):

https://flutter-docs-prod--docs-pr13979-remove-const-perf-reco-n1k77du1.web.app

[abdallahshaban557]

LGTM