Follow-up the PR #3631(Encapsulate Stepper, Planner, Endstops in singleton classes)

plan_bed_level_matrix -> planner.bed_level_matrix in multi extruders section of Marlin_main.cpp
It probably fix the Issue #3658(plan_bed_level_matrix not declarate).

movesplanned() -> planner.movesplanned() in ADVANCED_OK section of Marlin_main.cpp
It fix compilation error when ADVANCED_OK is enabled

The fix is simple. Most changes are because of changed indendation,
bacause of leaving early for wrong tool number

- Use new `DEBUG_POS` macro for `DELTA` debug
- Neaten up `prepare_move` a smidgen
- Remove an old commented `prepare_move()` line

Signed-off-by: Jeff K

As noted by @snowzach in #3461

Also removes the re-definition of now inside M190's while loop