Fixed the issue causing flags cleared by calling +[Bugsnag clearFeatureFlagWithName:] to appear in crash reports

[robert-smartbear]

Goal

Due to a bug in BSGAtomicFeatureFlagStore, feature flags that are cleared by name were not removed completely and would appear in crash reports

Changeset

Fixed the implementation of BSGAtomicFeatureFlagStore.

Testing

E2E tests

[github-actions]

Bugsnag.framework binary size increased by 400 bytes from 669,160 to 669,560

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +220  +0.1%    +220    __TEXT,__text
  +0.1%    +208  +0.1%    +208    Symbol Table
  +0.1%    +160  +0.1%    +160    String Table
  +1.1%     +32  +1.1%     +32    Binding Info
  +0.9%     +32  +0.9%     +32    __DATA,__const
  +0.2%     +32  +0.2%     +32    __TEXT,__objc_stubs
  +0.1%     +18  +0.1%     +18    __TEXT,__objc_methname
  [ = ]       0  +0.2%     +16    __DATA,__bss
  +1.1%      +8  +1.1%      +8    __DATA,__objc_classrefs
  +0.2%      +8  +0.2%      +8    __DATA,__objc_selrefs
  -0.8%     -48  -0.5%     -64    [__DATA]
  -1.4%    -270  -1.4%    -270    [__TEXT]
  [ = ]       0 -15.5%    -400    [__LINKEDIT]
  +0.1%    +400  [ = ]       0    TOTAL

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