remove `b` shorthand for barns

This is too easily conflicting with a shorthand for `bit` from computer science or `bar` from GNU Units. Still keep the addition of metric prefixes.

numbat/modules/units/misc.nbt

@@ -10,7 +10,7 @@ unit bar: Pressure = 100 kPa
 unit angstrom: Length = 1e-10 meter
 
 @metric_prefixes
-@aliases(barns, b: short)
+@aliases(barns)
 unit barn: Area = 1e-28 meter^2
 
 @metric_prefixes