CONTRIBUTING.md refers to benchmarking script which no longer exists

[kaisalmon]

After this pull request #2495 the benchmarking has been removed. This was due to the changes in #2330

This is about:

• A typo
• Innaccurate/misleading documentation (e.g. technically incorrect advice)
• Undocumented code
• Outdated documentation
• Other

Problem
File: https://github.com/yewstack/yew/blob/master/CONTRIBUTING.md

The documentation for the contribution requirements was written for a previous version of yew which had the benchmark makescript.

Details about the solution you'd like

Proposed solution:
Change

## Benchmarks

If you wish to improve the performance of Yew, we ask you to prove the improvements of your changes through benchmarking.

Some components of Yew have dedicated benchmarks which can be run with the following command:

` ``bash
cargo make benchmarks
`` `

There's also a benchmark for the framework as a whole.
Simply clone [bakape/js-framework-benchmark](https://github.com/bakape/js-framework-benchmark)
and follow the repository's README.

Feel free to add new benchmark tests if the current benchmark coverage is insufficient!

> See #1453 for a discussion on how to make this easier.

to simply

## Benchmarks
js-framework-benchmark is used as a benchmark for the framework as a whole.
Simply clone [bakape/js-framework-benchmark](https://github.com/bakape/js-framework-benchmark)
and follow the repository's README.

Additional context (Optional)

• Do we have any intentions to put the benchmarking back in? If so, should we mention that?
• Should we explicitly mention the lack of benchmarking?
• What is the current policy on performance improvements, since "If you wish to improve the performance of Yew, we ask you to prove the improvements of your changes through benchmarking." no longer makes sense

Questionaire (Optional)

• I'd like to write this documentation
• I'd like to write this documentation but I'm not sure what's needed
• I don't have time to add this right now, but maybe later