Sketcher: Fix increment of parabola constraint tag

It is an invisible and unstated assumption of the redundant-constraint
removal code that the "tag" of a constraint can be used as the index
into the SketchObject's Constraint list. This is true for all objects
except a parabola, which, when creating its internal alignment focus
constraints, incremented the ConstraintCounter for *each* of the
internal constraints, instead of only once.

src/Mod/Sketcher/App/Sketch.cpp

@@ -4162,8 +4162,6 @@ int Sketch::addInternalAlignmentParabolaFocalDistance(int geoId1, int geoId2)
 
         int tag = ++ConstraintsCounter;
         GCSsys.addConstraintP2PCoincident(p1, vertexpoint, tag);
-
-        tag = ++ConstraintsCounter;
         GCSsys.addConstraintP2PCoincident(p2, focuspoint, tag);
 
         return ConstraintsCounter;