Removes unused parameter

[rafaparadela]

With the only purpose of doing a major refactoring to replace the free-monad approach in favor of tagless-final (a more convenient pattern), I've been analyzing the pieces that need to be re-implemented and I realized that the parameter C in this class HttpClient[C, M[_]] is completely unused and also forces to propagate it up several layers, affecting even to the way we use this library.

This PR removes consequently this unnecessary parameter across the board.

Note: I'm assuming that at some point, we were using it but when discarded the ScalaJS compatibility stopped making sense anymore. Now, it was always resolved as HttpResponse[String].

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@f925fd8). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #327   +/-   ##
=========================================
  Coverage          ?   87.85%           
=========================================
  Files             ?       37           
  Lines             ?      700           
  Branches          ?        5           
=========================================
  Hits              ?      615           
  Misses            ?       85           
  Partials          ?        0

Impacted Files	Coverage Δ
...cala/github4s/HttpRequestBuilderExtensionJVM.scala	100% <ø> (ø)

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[BenFradet]

This was an artifact of the scalajs implementation iirc, nice to get rid of it 👍