refactor: remove computation from LegacyRoomManager

[ggazzo]

https://rocketchat.atlassian.net/browse/ARCH-1684

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[dionisio-bot]

Looks like this PR is not ready to merge, because of the following issues:

• This PR is targeting the wrong base branch. It should target 7.9.0, but it targets 7.8.0

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[codecov]

Codecov Report

Attention: Patch coverage is 72.28916% with 23 lines in your changes missing coverage. Please review.

Project coverage is 64.79%. Comparing base (d4226a5) to head (8a3c50a).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #36135      +/-   ##
===========================================
- Coverage    64.79%   64.79%   -0.01%     
===========================================
  Files         3153     3153              
  Lines       104876   104859      -17     
  Branches     19985    20019      +34     
===========================================
- Hits         67955    67943      -12     
+ Misses       34242    34228      -14     
- Partials      2679     2688       +9     

Flag	Coverage Δ
unit	69.53% <ø> (+0.01%)	⬆️

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