Tweak EitherT.pure and rightT to avoid a map call

[joroKr21]

It's not necessary. Closes #3398

[codecov-io]

Codecov Report

Merging #3408 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3408   +/-   ##
=======================================
  Coverage   92.70%   92.70%           
=======================================
  Files         379      379           
  Lines        7981     7981           
  Branches      230      228    -2     
=======================================
  Hits         7399     7399           
  Misses        582      582           

Flag	Coverage Δ
#scala_version_212	92.73% <100.00%> (ø)
#scala_version_213	92.49% <100.00%> (ø)

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/EitherT.scala	98.50% <100.00%> (ø)

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[LukaJCB]

Looks good to me!

[kailuowang]

LGTM thanks!