The regression was introduced by the new FDM in 3.0.8; the properties
/surface-positions/{elevator,rudder}-pos-norm were fixed at zero, so
the surfaces no longer moved. This change fixes that.
* MiG-15bis.xml (fcs/pitch-trim-sum-clipped-scaled): rename to
fcs/elevator-pos-norm.
(fcs/rudder-trim-sum-scaled): rename to fcs/rudder-pos-norm.
(fcs/rudder-pos-deg_1, fcs/rudder-pos-rad_1): remove "_1" suffix.
