Kill scala-collection-compat

[travisbrown]

Are we sure we even need this? I just ran buildJVM on my laptop for 2.13.0-M4 and it worked just fine.

[kailuowang]

Just went through the original PR adding M4 support, it is not needed indeed.
What happened was that there were a lot of deprecations in collections whose migration can't be cross compiled without scala-collection-compat. it turned out that even with scala-collections-compat it was still too much work, so we ended up disabling the deprecation warnings in 2.13-M4 build and roll back those changes. The dependency, however, was left alone.

Once build green I am going to merge this one and cherry-pick it back to 1.3.x branch and release a 1.3.2.
Thanks @travisbrown!

[LukaJCB]

👍

[travisbrown]

Thanks much @kailuowang, @LukaJCB!

[codecov-io]

Codecov Report

Merging #2488 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2488   +/-   ##
=======================================
  Coverage   95.38%   95.38%           
=======================================
  Files         357      357           
  Lines        6517     6517           
  Branches      279      282    +3     
=======================================
  Hits         6216     6216           
  Misses        301      301

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