Remove need to define unused pins as -1 in pins files

[thinkyhead]

This is a cleanup that is both obvious and long overdue.

Background: Pins files are "littered" with definitions of pins they don't have, and #undef lines only needed due to certain choices in pins.h.

This PR removes instances in pins files where:

• Pins are defined as -1 (when they are not overrides of RAMPS pins, for example)
• Adds conditional defines to pins.h defining pins not defined by the board.
• Removes the pre-defined pins from the top of pins.h that required board-pins to #undef them.

This PR additionally:

• Adds microstepping and extruder auto fans to the sensitive pins list
• Adds FAN1 and FAN2 to the sensitive pins list
• Moves the conditional definition of analogInputToDigitalPin to pins.h
• Alters analogInputToDigitalPin to account for -1 (don't convert it into 0x9F!)