mc_pos_control: replace legacy refactor

Until now we replaced legacy position controller functionality inside
the flight task architecture to split up the huge position control
module into the different tasks and have a modular setup with a clear
setpoint interface. This commit removes all the legacy code and hard
switches to using the flight task architecture for multicopter.

This is done because maintaining and testing everything in parallel is
not sustainable. The architecture is by now tested to cover all basic
legacy functionality and missing corner cases can be fixed a lot easier
with the new architecture.