Murisi/remove reverse conversions (backport #4290)

[mergify]

Describe your changes

An alternative to #4288 where the ability to choose the epoch to which amounts are exchanged to is simply removed. This functionality became redundant once pinned transactions were removed in #3142 . More generally, this functionality is only needed for properly implementing functionality like account statements and pinned transactions where the amounts that were received need to be printed out exactly as they were in the past without any influence from conversions in the current tree.

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:

---

This is an automatic backport of pull request #4290 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

Attention: Patch coverage is 98.27586% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.18%. Comparing base (37ccfdb) to head (ff6c1d7).
Report is 9 commits behind head on maint-libs-0.47.

Files with missing lines	Patch %	Lines
crates/core/src/masp.rs	95.23%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           maint-libs-0.47    #4334      +/-   ##
===================================================
- Coverage            74.19%   74.18%   -0.02%     
===================================================
  Files                  345      345              
  Lines               110677   110623      -54     
===================================================
- Hits                 82118    82066      -52     
+ Misses               28559    28557       -2     

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