chore: clean up vendor dir after vendor_pkgs()

[albersonmiranda]

This PR adds a clean up step at the end of vendoring process to remove src/rust/vendor dir.

Closes #452

[CGMossa]

I like it. I have come to like the presence of the vendor directory even in usual development.
Could you add this folder to the rbuildignore and the gitignore too?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.69%. Comparing base (5d3ae4a) to head (215ec1a).

Additional details and impacted files

Files with missing lines	Coverage Δ
R/cran-compliance.R	84.41% <100.00%> (+0.63%)	⬆️

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[albersonmiranda]

Could you add this folder to the rbuildignore and the gitignore too?

@CGMossa use_extendr() already does it!