Remove sbt-hydra for now

[travisbrown]

See #2870 and my comment on #2848:

I wish this change had been put forward for more public discussion before being merged. I personally didn't hear anything about this before seeing all the tweets about it after it was done.

I think there are many good reasons to avoid requiring a closed-source sbt plugin to run the Cats build, and that's even if it had been tested thoroughly and we had established that it wasn't likely to cause problems. As it is right now running sbt prePR from a clean checkout crashes with this error on both my Mac laptop and Linux desktop, and it's completely impossible to investigate because sbt-hydra is just a binary from a commercial repo.

I appreciate Triplequote's generosity, but the Cats build already has plenty of sharp edges, especially for new contributors, and I really don't think throwing a WIP closed-source sbt plugin into the mix was a good idea.

If we're going to use a closed-source sbt plugin in the Cats build, I think there's a pretty clear case for it being gated behind an environmental variable (Guillaume Martres pointed out this example for sbt-dotty). This PR doesn't do that—it just disables the plugin for now so that when we tell people to run sbt prePR they don't get an opaque error message.

[LukaJCB]

Environment variable sounds like a better solution to be too 👍

[codecov-io]

Codecov Report

Merging #2871 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2871   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files         368      368           
  Lines        6948     6948           
  Branches      307      308    +1     
=======================================
  Hits         6546     6546           
  Misses        402      402

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[kailuowang]

I totally agree that we shouldn't force contributors to use hydra.
Hydra is only used on cats build on main repo branches (i.e, it is not used in PR builds or when run locally)
sbt-hydra 2.1.4 was doing just that. I think all we need to do is to revert back to sbt-hydra 2.1.4

[travisbrown]

@kailuowang The objection isn't just to Hydra being required, it's sbt-hydra, which is also not open source. If it's behind a gate and not the default, I don't personally have a problem with it being in the build, but I don't think it should be required.