Cleanup partial retrieval codepaths ( zero functional changes )

[ribasushi]

• Adjust the ExportRef comment to reflect change from Chore/dm level tests plus merkle proof cars #7673
• Simplify control flow in parseDagSpec() ( read diff under -w )

[codecov]

Codecov Report

Merging #7688 (d5b4d93) into master (ab55a6f) will decrease coverage by 0.08%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #7688      +/-   ##
==========================================
- Coverage   39.56%   39.48%   -0.09%     
==========================================
  Files         637      637              
  Lines       67924    67924              
==========================================
- Hits        26877    26817      -60     
- Misses      36445    36489      +44     
- Partials     4602     4618      +16     

Impacted Files	Coverage Δ
api/types.go	52.77% <ø> (ø)
node/impl/client/client.go	49.69% <66.66%> (-0.25%)	⬇️
node/modules/dtypes/mpool.go	87.50% <0.00%> (-12.50%)	⬇️
blockstore/api.go	24.00% <0.00%> (-8.00%)	⬇️
blockstore/blockstore.go	62.96% <0.00%> (-7.41%)	⬇️
chain/events/observer.go	71.64% <0.00%> (-6.72%)	⬇️
chain/stmgr/execute.go	86.95% <0.00%> (-4.35%)	⬇️
chain/stmgr/call.go	71.51% <0.00%> (-3.64%)	⬇️
chain/sub/incoming.go	54.43% <0.00%> (-3.38%)	⬇️
node/impl/full/mpool.go	25.21% <0.00%> (-1.74%)	⬇️
... and 17 more

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[ribasushi]

@magik6k just a reminder: we should still fix the export-to-car effectively not working as-is