core/aggsigdb: remove unused MemDBV2

[KaloyanTanev]

Removes the unused MemDBV2 aggsigdb implementation and its aggsigdb_v2 feature flag. Introduced in #2950 as a simpler alternative to the channel-based MemDB, it has sat at statusAlpha for ~2 years and never ran in production (the default --feature-set=stable never enables it), while doubling the maintenance cost of every aggsigdb change.

category: refactor
ticket: none

[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 65.39%. Comparing base (67f4b59) to head (d5d4a37).

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4695      +/-   ##
==========================================
- Coverage   65.45%   65.39%   -0.07%     
==========================================
  Files         247      246       -1     
  Lines       30057    29984      -73     
==========================================
- Hits        19675    19607      -68     
+ Misses      10381    10376       -5     
  Partials        1        1              

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