Drop ./x suggest

[jieyouxu]

This PR removes the current ./x suggest implementation (#109933, #106249) and associated docs for several reasons:

1. Primarily, ./x suggest is another "flow" in bootstrap that incurs extra complexity and more invariants that bootstrap has to maintain. This causes more friction when trying to investigate and fix staging problems. As far as I know, this flow has not been actively maintained in quite a while, and I'm not aware of interest in maintaining it. Bootstrap really could use less implementation complexity with a very limited maintenance bandwidth.
2. The current ./x suggest implementation "bypasses" the usual stage defaults for the various check/build/test/etc. flows, and it's not really possible to have a stage default because ./x suggest --run produces a sequence of suggestions like [./x check, ./x test library/std, ..] and then tries to run all of them in sequence, based on which files are modified.
3. We've not seen a lot of interest both in using it or extending static/dynamic test suggestions. Last extensions were Add x suggest entries for testing mir-opt and coverage #117961 and Suggest pattern tests when modifying exhaustiveness #120763. I'm not convinced the extra implementation complexity is worth it. This was discussed in:
   • #t-infra/bootstrap > Dropping the current `./x suggest` flow implementation
   • #t-compiler > Dropping current `./x suggest` implementation

Closes #109933 (the current implementation is being removed).
Closes #143569 (by removing ./x suggest altogether).

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[rustbot]

triagebot.toml has been modified, there may have been changes to the review queue.

cc @davidtwco, @wesleywiser

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

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