Avoid updating merkle paths of spent notes

[sug0]

Describe your changes

Avoid updating merkle paths of spent notes. This should optimize the synchronous path of the shielded sync on the ledger client.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.91%. Comparing base (b2332bf) to head (550e03a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4018      +/-   ##
==========================================
- Coverage   73.92%   73.91%   -0.01%     
==========================================
  Files         341      341              
  Lines      106510   106520      +10     
==========================================
- Hits        78740    78739       -1     
- Misses      27770    27781      +11     

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