Remove continuous benchmarking infrastructure

[DavisVaughan]

We are planning on removing the continuous benchmarking infrastructure from bench. This decision is being made for a few reasons:

• It is only partially finished, as COVID interrupted development and we haven't picked it back up.
• touchstone exists, and I think the relative benchmark ideas that it uses are quite neat. Arrow also has arrowbench and their larger language agnostic conbench service, which is worth exploring.
• I am not completely convinced that git notes are the best long term storage for results. It doesn't seem widely supported by git (they no longer even show notes in the commit message by default), and they are kind of a pain to work with if you have to make any manual adjustments to the notes since they are technically orphaned branches. It is also hard to actually see the files on github. You can locally checkout refs/notes/benchmarks and that shows them to you.
• A big reason is because none of us are actively using this infrastructure yet, with the exception of tibble.
• Because we aren't convinced that these functions are stable, we haven't been shipping them in CRAN releases, which makes doing a bench release a little complicated (you have to remove, send to CRAN, and then re-add the CB functions)

It has been in the dev version for awhile, so I am going to park the current implementation in a branch. tibble can use this if it needs some temporary support. That branch is frozen/continuous-benchmarking https://github.com/r-lib/bench/tree/frozen/continuous-benchmarking.

There is also some additional work in https://github.com/r-lib/bench/tree/CB where development was pushed forward a little further, but was never merged in.

It would be nice to figure out a way to pick this back up in the future, in particular for rlang, vctrs, and dplyr's vector functions like case_when() and first(). Having benchmarks in place for those lower level tools could have likely avoided some of the patches needed in dplyr 1.1.0 related to performance issues.