drop invocation of redismod_cluster docker

[dvora-h]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

We don't currently use it for testing and it causes CI fail.

[codecov-commenter]

Codecov Report

Merging #2307 (2f8fb3a) into master (f665bd3) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2307      +/-   ##
==========================================
+ Coverage   92.05%   92.06%   +0.01%     
==========================================
  Files         109      109              
  Lines       28134    28134              
==========================================
+ Hits        25899    25902       +3     
+ Misses       2235     2232       -3     

Impacted Files	Coverage Δ
tests/conftest.py	86.50% <0.00%> (-0.80%)	⬇️
redis/asyncio/connection.py	85.57% <0.00%> (+0.11%)	⬆️
tests/test_cluster.py	97.08% <0.00%> (+0.11%)	⬆️
tests/test_asyncio/test_cluster.py	97.42% <0.00%> (+0.13%)	⬆️

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