fix: call proper flush method when entering background.

[sunitaprajapati89]

• Refer ticket - https://twilio-engineering.atlassian.net/browse/LIBRARIES-2112
• Removed dead flush method call from enterBackground()

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.07%. Comparing base (7d47f4b) to head (607b902).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/Segment/Plugins/SegmentDestination.swift	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #389      +/-   ##
==========================================
- Coverage   70.09%   70.07%   -0.03%     
==========================================
  Files          48       48              
  Lines        4795     4795              
==========================================
- Hits         3361     3360       -1     
- Misses       1434     1435       +1     

Files with missing lines	Coverage Δ
Sources/Segment/Plugins/SegmentDestination.swift	90.90% <0.00%> (ø)

... and 1 file with indirect coverage changes

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