Remove focus feature flag

[ThibautGeriz]

Motivation

Activate foreground periods issue for all our user.

Analysis: https://docs.google.com/document/d/17FsROHTNE8hh_Ra6blCgNwyZn8m2PqRefS_WIWjfBSk/edit#

Changes

• remove FF
• remove monitoring

[codecov-commenter]

Codecov Report

Merging #1053 (50d813e) into main (f61c866) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1053      +/-   ##
==========================================
+ Coverage   89.01%   89.10%   +0.09%     
==========================================
  Files          87       87              
  Lines        4123     4112      -11     
  Branches      950      947       -3     
==========================================
- Hits         3670     3664       -6     
+ Misses        453      448       -5     

Impacted Files	Coverage Δ
packages/rum-core/src/boot/startRum.ts	37.03% <100.00%> (ø)
packages/rum-core/src/domain/foregroundContexts.ts	90.76% <100.00%> (+5.24%)	⬆️

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