chore(metrics): remove first custom metric sent message

[obostjancic]

Follow up of #71096

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.85%. Comparing base (4c8d765) to head (7370a38). Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #71335      +/-   ##
==========================================
+ Coverage   76.91%   77.85%   +0.94%     
==========================================
  Files        6532     6533       +1     
  Lines      291002   291120     +118     
  Branches    50354    50379      +25     
==========================================
+ Hits       223826   226661    +2835     
+ Misses      60803    58226    -2577     
+ Partials     6373     6233     -140     

Files	Coverage Δ
src/sentry/receivers/onboarding.py	83.33% <ø> (+2.40%)	:arrow_up:

... and 212 files with indirect coverage changes