Remove the STEP_DELAY Kconfig option and replace it with a per-stepper
step_pulse_duration printer.cfg config option.
The AVR code will continue to have optimized code to step and "unstep"
in the same function (which is automatically activated when the step
delay is 40 ticks or less). This change removes the Kconfig option
for single function step/unstep on 32bit processors.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>