Remove unused type parameter for Parallel instances

[travisbrown]

This PR includes @Billzabob's commit from #2789 but rebased against current master, where the change has to happen in two places, and slightly adjusted to avoid breaking binary compatibility.

I missed @Billzabob's PR the first time around but should have caught this in #3012.

The change does technically break source compatibility, but (while it fixes a real problem) it's also pretty minor, so I'm not worried about it causing anyone problems. I personally think we can / should try to get it into 2.0.0 today.

[travisbrown]

The failing 2.13 test is unrelated, so I restarted the job. I'm not sure whether it needs attention:

[info] - CommutativeGroup[BigDecimal].commutativeGroup.associative *** FAILED ***
[info]   GeneratorDrivenPropertyCheckFailedException was thrown during property evaluation.
[info]    (Discipline.scala:12)
[info]     Falsified after 8 successful property evaluations.
[info]     Location: (Discipline.scala:12)
[info]     Occurred when passed generated values (
[info]       arg0 = 1.4502366979631793E-19,
[info]       arg1 = 3.2509612437180712E-9,
[info]       arg2 = -81805312727198.39
[info]     )
[info]     Label of failing property:
[info]       Expected: -81805312727198.38999999674903875613
[info]   Received: -81805312727198.38999999674903875614

[codecov-io]

Codecov Report

Merging #3042 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3042      +/-   ##
==========================================
- Coverage   93.46%   93.43%   -0.03%     
==========================================
  Files         368      368              
  Lines        6975     6975              
  Branches      187      187              
==========================================
- Hits         6519     6517       -2     
- Misses        456      458       +2

Impacted Files	Coverage Δ
testkit/src/main/scala/cats/tests/Helpers.scala	96% <0%> (-4%)	⬇️
...cala/cats/kernel/instances/FunctionInstances.scala	96.96% <0%> (-3.04%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9dce008...70db92e. Read the comment docs.

[LukaJCB]

👍

[kailuowang]

I am going to wait for Oscar until this afternoon. but I think it's such a trivial one that we can safely merge with 2 maintainer approvals.

[johnynek]

Personally I’m far more worried about binary incompatibility because they show up at runtime when linking large sets of dependencies together.

Source incompatibility isn’t great, but can be detected with a compiler and doesn’t hit you at runtime. Also, for almost all users, they won’t see this because 1. Parallel isn’t that commonly used, 2. I imagine they are resolving this method implicitly.