chore: cleanup unused reporting client code after migrating to commonclient

[satishrudderstack]

Description

• We are currently using the common reporting client for flushing the metrics to Reporting Service. As part of this PR, we are cleaning up un-used code.

Linear Ticket

Fixes OBS-914

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.76%. Comparing base (9017582) to head (73bf92f).

Files with missing lines	Patch %	Lines
enterprise/reporting/error_reporting.go	50.00%	1 Missing ⚠️
enterprise/reporting/flusher/flusher.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6385      +/-   ##
==========================================
+ Coverage   77.64%   77.76%   +0.11%     
==========================================
  Files         523      523              
  Lines       70313    70125     -188     
==========================================
- Hits        54593    54531      -62     
+ Misses      12879    12772     -107     
+ Partials     2841     2822      -19     

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[itsmihir]

we can remove this as well

[itsmihir]

LGTM, added one small comment