Remove non-helping stills merge/scaling knobs

Trims three opt-in stills parameters that did not improve data quality on the
external-reference (PDB R-free) battery and only added code:

- --partiality-uncertainty: the (1-p)/p merge-sigma term was null on KR2/OCP/
  NmHR/CTX vs their reference structures (and neutral-to-harmful at higher
  coefficients); removed the flag, setting and CorrectedSigma term.
- --stills-modulation: the detector-plane flat-field surface was net-negative
  on flooded data; removed the flag, setting and MergeOnTheFly::RefineModulation
  (the rotation modulation in RotationScaleMerge is unaffected).
- --min-indexed-fraction: every value other than the 0.20 default collapsed
  CC1/2; removed the override flag/setter, keeping the fixed 0.20 acceptance
  floor.

Default behaviour is unchanged (all three were off / at their default).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/IndexingSettings.cpp

@@ -28,11 +28,6 @@ int64_t IndexingSettings::GetViableCellMinSpots() const {
     return viable_cell_min_spots;
 }
 
-IndexingSettings &IndexingSettings::MinIndexedSpotFraction(float input) {
-    min_indexed_spot_fraction = input;
-    return *this;
-}
-
 float IndexingSettings::GetMinIndexedSpotFraction() const {
     return min_indexed_spot_fraction;
 }