Use unit from Applicative

[johnynek]

We have a unit on Applicative, but we weren't using it in a few places where we could.

[LukaJCB]

👍

[codecov-io]

Codecov Report

Merging #2246 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2246   +/-   ##
=======================================
  Coverage   94.95%   94.95%           
=======================================
  Files         333      333           
  Lines        5787     5787           
  Branches      222      218    -4     
=======================================
  Hits         5495     5495           
  Misses        292      292

Impacted Files	Coverage Δ
core/src/main/scala/cats/Alternative.scala	88.88% <100%> (ø)	⬆️
core/src/main/scala/cats/Applicative.scala	95% <100%> (ø)	⬆️

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