[layouts] Simplify scalebar unit handling

Instead of auto setting the units to meters/feet, and then using
the label multiplier setting to obtain a scalebar in km/miles,
instead we always keep auto-set label multipliers at 1 and just
always keep the users explicitly set scale bar units.

This should help avoid the complexity of layout scale bars, while
still keeping flexibility if someone does need custom units.

src/core/layout/qgslayoutitemscalebar.cpp

@@ -400,48 +400,8 @@ void QgsLayoutItemScaleBar::applyDefaultSize( QgsUnitTypes::DistanceUnit units )
     double initialUnitsPerSegment = widthInSelectedUnits / 10.0; //default scalebar width equals half the map width
     mSettings.setUnitsPerSegment( initialUnitsPerSegment );
 
-    switch ( units )
-    {
-      case QgsUnitTypes::DistanceUnknownUnit:
-      {
-        upperMagnitudeMultiplier = 1.0;
-        setUnitLabel( tr( "units" ) );
-        break;
-      }
-      case QgsUnitTypes::DistanceMeters:
-      {
-        if ( initialUnitsPerSegment > 1000.0 )
-        {
-          upperMagnitudeMultiplier = 1000.0;
-          setUnitLabel( tr( "km" ) );
-        }
-        else
-        {
-          upperMagnitudeMultiplier = 1.0;
-          setUnitLabel( tr( "m" ) );
-        }
-        break;
-      }
-      case QgsUnitTypes::DistanceFeet:
-      {
-        if ( initialUnitsPerSegment > 5419.95 )
-        {
-          upperMagnitudeMultiplier = 5419.95;
-          setUnitLabel( tr( "miles" ) );
-        }
-        else
-        {
-          upperMagnitudeMultiplier = 1.0;
-          setUnitLabel( tr( "ft" ) );
-        }
-        break;
-      }
-
-      default:
-        setUnitLabel( QgsUnitTypes::toAbbreviatedString( units ) );
-        upperMagnitudeMultiplier = 1;
-        break;
-    }
+    setUnitLabel( QgsUnitTypes::toAbbreviatedString( units ) );
+    upperMagnitudeMultiplier = 1;
 
     double segmentWidth = initialUnitsPerSegment / upperMagnitudeMultiplier;
     int segmentMagnitude = std::floor( std::log10( segmentWidth ) );