Completely remove experimental GRPC module

[olegbespalov]

What?

This change completely removes the experimental GRPC module, as it was announced in #3490

Why?

The same functionality is in k6-core, and we waited for several releases.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

#3490

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.64%. Comparing base (b80ad9f) to head (76e1cc8).

❗ Current head 76e1cc8 differs from pull request most recent head 25d12ad. Consider uploading reports for the commit 25d12ad to get more accurate results

Files	Patch %	Lines
js/jsmodules.go	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3530      +/-   ##
==========================================
+ Coverage   73.62%   73.64%   +0.02%     
==========================================
  Files         277      277              
  Lines       20243    20240       -3     
==========================================
+ Hits        14904    14906       +2     
+ Misses       4394     4390       -4     
+ Partials      945      944       -1     

Flag	Coverage Δ
macos	73.56% <71.42%> (?)
ubuntu	73.59% <71.42%> (+0.03%)	⬆️
windows	73.48% <71.42%> (+0.02%)	⬆️

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