The current z_hop support did not work correctly with common slicers
that change Z height between retract and unretract, and it did not
work correctly with slicers that issue a retract at the end of a print
and then at the beginning of the next print.
The z_hop support is therefore being removed until these issues can be
fixed.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>