Fix, cleanup DELTA G28 / G29 support functions

[thinkyhead]

Reference: #4361 (comment)

Recent changes to probing and movement functions missed some nuances, such as:

• How to calculate a difference in position. (Use subtraction)
• How to use the movement functions. (Don't set current_position ahead of calling movement functions except those that specifically move to the current_position because some of them want to compare the destination to current_position.)
• There's no need to set destination from current_position after a call to prepare_move_to_destination* because they always call set_current_to_destination.

This PR patches these bugs and removes other superstitious redundancies in setting current_position and destination.

[ghost]

Thanks for the trying to fix and hard work, but results of G28 and G29 of DELTA has been exactly same as Yesterday's report unfortunately...

[ghost]

By the way, your (@thinkyhead's) DELTA machine has been shipped by DHL just now.
So, Is it better that fixing for homing and probing of DELTA are left as it's until you get a DELTA machine?

[thinkyhead]

shipped by DHL just now

❗ WOW❗ I'm very excited for its arrival! But I think I should keep addressing any issues that I happen to catch. For example, #4370 brings some important changes and certainly addresses some bugs. That PR deals with the movement functions mostly, not with the logic of delta probing (yet). So I need to look at the logic next.

If you do more testing on your delta after #4370 is merged, please use DEBUG_LEVELING_FEATURE + M111 S32 and post a log. That is very helpful, even more than a video oftentimes.

[ghost]

Oh, I've never known about M111 S32.
I wasn't sure why I can't get debugging log nevertheless I enabled DEBUG_LEVELING_FEATURE and uint8_t marlin_debug_flags = DEBUG_LEVELING.
I'm thankful for teaching.

[thinkyhead]

I've never known about M111 S32.

Yes, we need to mention that in the configuration file! (#4378)