chore: delete IsLastLID

[cheb0]

Description

It's easier to just use a new Source interface to iterate over all postings in index analyzer

---

• I have read and followed all requirements in CONTRIBUTING.md;
• I used LLM/AI assistance to make this pull request;

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.58%. Comparing base (8d40146) to head (abc9f69).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/index_analyzer/main.go	0.00%	43 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #469      +/-   ##
==========================================
+ Coverage   70.45%   70.58%   +0.12%     
==========================================
  Files         232      232              
  Lines       18364    18316      -48     
==========================================
- Hits        12939    12928      -11     
+ Misses       4450     4413      -37     
  Partials      975      975              

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