Don't trigger discarded_notification_center_observer when the observer is returned

[marcelofabri]

Fixes #1525

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Codecov Report

Merging #1581 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1581      +/-   ##
==========================================
+ Coverage   84.24%   84.28%   +0.03%     
==========================================
  Files         192      192              
  Lines        9979    10000      +21     
==========================================
+ Hits         8407     8428      +21     
  Misses       1572     1572

Impacted Files	Coverage Δ
...ules/DiscardedNotificationCenterObserverRule.swift	100% <100%> (ø)	⬆️

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[jpsim]

Nice fix, although I worry that there are many more cases in which a valid use of addObserver that doesn't store to a variable or return, such as appending the result to an Array/Set/etc, among many other operations...