stepper: Make step pulse duration customizable at run-time

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>

src/Kconfig

@@ -73,31 +73,6 @@ config USB_SERIAL_NUMBER
     string "USB serial number" if !USB_SERIAL_NUMBER_CHIPID
 endmenu
 
-# Step timing customization
-config CUSTOM_STEP_DELAY
-    bool "Specify a custom step pulse duration"
-    depends on LOW_LEVEL_OPTIONS
-config STEP_DELAY
-    int
-    default 2
-config STEP_DELAY
-    int "Step pulse duration (in microseconds)"
-    depends on CUSTOM_STEP_DELAY
-    help
-        Specify the duration of the stepper step pulse time. This
-        setting applies to all stepper drivers controlled by the
-        micro-controller. If this value is set to zero (or less) then
-        the code will "step" and "unstep" in the same C function.
-
-        A setting of zero (or less) on 8-bit AVR micro-controllers
-        results in a minimum step pulse time a little over 2us.
-
-        A setting of zero on ARM micro-controllers typically results
-        in a minimum step pulse time of 20 cpu cycles.
-
-        The default for AVR is -1, for all other micro-controllers it
-        is 2us.
-
 config INITIAL_PINS
     string "GPIO pins to set at micro-controller startup"
     depends on LOW_LEVEL_OPTIONS