CI: promote unused linter

[cce]

Summary

In #6105 (comment) after the "unused" linter noticed some unused peer latency metrics related to the ping handler (left unused in #3028 and replaced partly by #4745), @algorandskiy recommended we remove the rest of the unused code in a separate PR.

This promotes the "unused" linter from the reviewdog-warnings to the reviewdog-errors CI job and cleans up the remaining unused lint issues.

Test Plan

Existing tests should pass.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 9 lines in your changes missing coverage. Please review.

Project coverage is 56.19%. Comparing base (1d08955) to head (e5286be).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
network/msgOfInterest.go	50.00%	4 Missing and 1 partial ⚠️
ledger/store/trackerdb/generickv/schema.go	0.00%	1 Missing ⚠️
...e/trackerdb/sqlitedriver/spVerificationAccessor.go	0.00%	1 Missing ⚠️
network/p2p/capabilities.go	0.00%	1 Missing ⚠️
shared/pingpong/accounts.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6120      +/-   ##
==========================================
- Coverage   56.20%   56.19%   -0.02%     
==========================================
  Files         494      494              
  Lines       69936    69914      -22     
==========================================
- Hits        39311    39288      -23     
+ Misses      27955    27948       -7     
- Partials     2670     2678       +8     

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