Fixing what appears to be missing X/Y logic in M251 command.

The implementation of setting the Z-height with the M251 S2 command
only accessed or set parameters pertaining to the Z-axis.  While in
the Cartesian system this is the case, the Delta mechanism doesn't
"think" that way. This commit adds (re-)setting of the X and Y axes in
order to fix the issues that were occurring due to inconsistent
{X,Y,Z}MaxSteps values.
Remove bad parameter reference in M251 command.