[docs] source subfolder is no longer used

[uilianries]

Remove source_subfolder from new recipe templates. It's no longer used, as layout is preferred.

---

• I've read the guidelines for contributing.
• I've followed the PEP8 style guides for Python code in the recipes.
• I've used the latest Conan client version.
• I've tried at least one configuration locally with the conan-center hook activated.

[conan-center-bot]

Updating docs!

[prince-chrismc]

There's more docs about this

https://github.com/conan-io/conan-center-index/blob/master/docs/reviewing.md#subfolder-properties

those should be removed?

[uilianries]

There's more docs about this

https://github.com/conan-io/conan-center-index/blob/master/docs/reviewing.md#subfolder-properties

those should be removed?

It should be updated. Removed in case using cmake_layout. Not all PRs are updating CMake flow, only mandatory tools asked by the linter.