Avoid updating merkle paths of spent notes (backport #4018)

[mergify]

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:

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This is an automatic backport of pull request #4018 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.05%. Comparing base (73c5d2a) to head (ced696f).
Report is 1 commits behind head on maint-0.45.

Additional details and impacted files

@@              Coverage Diff               @@
##           maint-0.45    #4051      +/-   ##
==============================================
- Coverage       74.06%   74.05%   -0.01%     
==============================================
  Files             341      341              
  Lines          106561   106571      +10     
==============================================
- Hits            78920    78919       -1     
- Misses          27641    27652      +11     

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