Bugfix Autolevel/Z-Probe-Sled

When moving to probe point the feedrate (in mm/min) is not converted to
mm/sec resulting in very rapid movement.

If autoleveling is enabled, dock_sled function is being generated but
requires SLED_DOCKING_OFFSET to be defined.
Bugfix: If SLED_DOCKING_OFFSET is not defined, assume a value of 0 and
drop it completely.
Improvement: Generate dock_sled function only if Z_PROBE_SLED is
defined.