core/infosync: remove dead proposal topic

[KaloyanTanev]

Removes the infosync proposal topic, which negotiates the cluster-wide supported proposal types every epoch but whose result has had no consumers since #1762 (Feb 2023). The builder-vs-full decision is driven entirely by static config, so topicProposal, Component.Proposals, app.ProposalTypes and the core.ProposalType constants are all dead code. Dropping the topic is backward compatible: the priority protocol scores each topic independently, so one sent by fewer than quorum peers simply drops out during a mixed-version rollout.

category: refactor
ticket: #4728

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.57%. Comparing base (7691f96) to head (e3d36d6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
core/infosync/infosync.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4729      +/-   ##
==========================================
+ Coverage   65.49%   65.57%   +0.08%     
==========================================
  Files         246      246              
  Lines       30123    30115       -8     
==========================================
+ Hits        19729    19748      +19     
+ Misses      10393    10366      -27     
  Partials        1        1              

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