Get rid of E_STARTSTOP_STEPS.

This meant to be a firmware-provided retract feature but was
never really supported by G-code generators. Without their support
(by issueing M101/M103), it's pretty hard to detect extrusion
pauses, so this feature simply has not future.

As this was on by default, it saves over 200 bytes binary size
in a default configuration.
Implement adaptive homing feedrates.

For now for X min only, but it works excellently already.
Tested quite a few combinations and raising acceleration
or endstop clearance raises homing feedrate just as expected.