Remove flaky gRPC tests

[joanlopez]

What?

It removes a few gRPC client tests.

Why?

Because by observing them running in CI, and based on what has been discussed in #3545 and #3551, it looks like these have a flaky nature that's not easy to fix, and we even have doubts about its utility, so better to remove them for now.

I also tried to replace the hard-coded certificates, as suggested in #3549, and in a similar way as in #3527, but in this case it's not as easy as that because the server we use for the these tests (HTTPMultiBin) heavily relies on httptest.NewTLSServer from Go stdlib, which manages its own set of certificates.
Indeed, that's the reason why we also have to use the example.com domain there.

So, I'd suggest to either better to discard that idea, or at the very least leave it for the future because it looks like it's going to imply a larger refactor.

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)

I guess it can close #3551

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ef1f214) 72.82% compared to head (c8e6c5e) 72.90%.

❗ Current head c8e6c5e differs from pull request most recent head 452779f. Consider uploading reports for the commit 452779f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3554      +/-   ##
==========================================
+ Coverage   72.82%   72.90%   +0.08%     
==========================================
  Files         278      278              
  Lines       20947    20944       -3     
==========================================
+ Hits        15254    15269      +15     
+ Misses       4715     4701      -14     
+ Partials      978      974       -4     

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ubuntu	72.90% <ø> (?)
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