surgical removal of x_o.setter

[Jocho-Smith]

This PR removes the optional x_o.setter function that just falls back to set_x. Since set_x will be removed in future releases, this simplification should make it more readable.

Local tests passed.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.02%. Comparing base (587f922) to head (b708598).
⚠️ Report is 20 commits behind head on gsoc-2026.

Files with missing lines	Patch %	Lines
sbi/samplers/vi/vi_divergence_optimizers.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           gsoc-2026    #1937      +/-   ##
=============================================
+ Coverage      87.99%   88.02%   +0.02%     
=============================================
  Files            144      144              
  Lines          13499    13557      +58     
=============================================
+ Hits           11879    11934      +55     
- Misses          1620     1623       +3     

Flag	Coverage Δ
fast	81.62% <50.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sbi/inference/potentials/base_potential.py	92.45% <ø> (-0.41%)	⬇️
sbi/utils/conditional_density_utils.py	93.20% <ø> (-0.05%)	⬇️
sbi/samplers/vi/vi_divergence_optimizers.py	84.69% <50.00%> (-0.33%)	⬇️

... and 2 files with indirect coverage changes

[dgedon]

looks good, thanks!