refactor(feature-activation): remove enable_usage

[glevco]

Depends on #908

Motivation

Since Feature Activation was enabled on mainnet on the previous PR, we can now remove the enable_usage setting, as its only purpose was making Feature Activation available on testnet but not mainnet.

Acceptance Criteria

• Make FeatureService mandatory on VertexVerifiers.
• Remove enable_usage property from Feature Activation settings, and update code accordingly.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ded4c79) 85.30% compared to head (67326fd) 85.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
- Coverage   85.30%   85.21%   -0.09%     
==========================================
  Files         285      285              
  Lines       22365    22361       -4     
  Branches     3369     3368       -1     
==========================================
- Hits        19078    19056      -22     
- Misses       2612     2627      +15     
- Partials      675      678       +3     

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The base branch was changed.