Remove redundant assignment to RedisCluster.nodes_manager.

[thebarbershop]

Pull Request check-list

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Description of change

Nothing grandiose, but can we just remove that line that keeps bugging me? I don't think assigning None to the attribute, only to immediately assign a NodeManager instance to the same one, has any meaningful effect, does it?

Of course I'm not adding this to my resume or anything.. just spotted the line while reading through and couldn't help but make a PR.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.31 ⚠️

Comparison is base (6c708c2) 92.31% compared to head (824a5d5) 92.01%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2620      +/-   ##
==========================================
- Coverage   92.31%   92.01%   -0.31%     
==========================================
  Files         115      115              
  Lines       29744    29742       -2     
==========================================
- Hits        27459    27367      -92     
- Misses       2285     2375      +90     

Impacted Files	Coverage Δ
redis/cluster.py	90.41% <ø> (-0.02%)	⬇️

... and 26 files with indirect coverage changes

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