Remove dead K"doc" handling code

[aviatesk]

K"doc" nodes are converted to Core.@doc macrocalls by preprocessed_green_children during SyntaxTree construction, so they never appear in the final tree. Remove the unreachable K"doc" branches from iterate_toplevel_tree and extract_toplevel_symbol!, along with the unused
extract_doc_symbol! function.

Closes #586.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.88%. Comparing base (d0e1261) to head (269bdd2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage   67.71%   67.88%   +0.17%     
==========================================
  Files          51       51              
  Lines        8307     8295      -12     
==========================================
+ Hits         5625     5631       +6     
+ Misses       2682     2664      -18     

Flag	Coverage Δ
JETLS.jl	67.88% <100.00%> (+0.17%)	⬆️

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