Remove Kernel GCV node

[albertpod]

This PR is intended to clean up ReactiveMP.jl slightly.

KernelGCV is a very specific "entity," largely untested.
Also, I doubt the rules are compatible with the actual ReactiveMP API.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.17 🎉

Comparison is base (5d2f840) 62.54% compared to head (57ef050) 62.72%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   62.54%   62.72%   +0.17%     
==========================================
  Files         199      194       -5     
  Lines        7385     7364      -21     
==========================================
  Hits         4619     4619              
+ Misses       2766     2745      -21     

Impacted Files	Coverage Δ
src/ReactiveMP.jl	75.86% <ø> (ø)

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