feat(event cache): remove events when forgetting a room

[bnjbvr]

Does what it says on the tin, and fixes a bug where clearing a room in the in-memory linked chunk would clear all rooms at once.

Part of #3280.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.39%. Comparing base (5548f38) to head (48df627).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4521   +/-   ##
=======================================
  Coverage   85.38%   85.39%           
=======================================
  Files         285      285           
  Lines       31961    31993   +32     
=======================================
+ Hits        27290    27320   +30     
- Misses       4671     4673    +2     

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[jmartinesp]

LGTM, thanks!