*: revert new v3 endpoint work

[pinebit]

Reverted all recent v3 endpoint work (that needed go-eth2-client fork with UniversalProposal).

This piece: #2885 shall remain present.

category: misc
ticket: none

[sonarcloud]

Quality Gate failed

Failed conditions
48.5% Duplication on New Code (required ≤ 30%)

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 75.49020% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 53.63%. Comparing base (53979d1) to head (51c286e).

Files	Patch %	Lines
testutil/validatormock/propose.go	65.07%	16 Missing and 6 partials ⚠️
core/fetcher/fetcher.go	89.28%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2939      +/-   ##
==========================================
+ Coverage   53.59%   53.63%   +0.04%     
==========================================
  Files         202      202              
  Lines       28864    28531     -333     
==========================================
- Hits        15469    15303     -166     
+ Misses      11439    11318     -121     
+ Partials     1956     1910      -46     

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