FFI: Remove UniFFI Swift mocks workaround now that we're using 0.31.0.

[pixlwave]

Following on from #6063, the workaround added in #5826 is no longer necessary as it happens automatically with mozilla/uniffi-rs#2720. This PR removes the workaround.

• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

Signed-off-by:

[pixlwave]

From what I can tell, xtask changes don't need/have a changelog entry, but let me know if I'm missing something.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.86%. Comparing base (179f38b) to head (090eb23).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6378   +/-   ##
=======================================
  Coverage   89.85%   89.86%           
=======================================
  Files         376      376           
  Lines      103093   103093           
  Branches   103093   103093           
=======================================
+ Hits        92638    92641    +3     
+ Misses       6878     6877    -1     
+ Partials     3577     3575    -2     

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[codspeed]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing doug/remove-uniffi-workaround (090eb23) with main (179f38b)}

[poljar]

Neat, less code.

[poljar]

From what I can tell, xtask changes don't need/have a changelog entry, but let me know if I'm missing something.

No, you're correct, xtask is an internal tool.

[pixlwave]

Neat, less code.

Umm yeah, so about that…

#6388