Default to AttitudePlus

We currently have an "Attitude" and "AttitudePlus" mode, the difference
being that AttitudePlus applies a trim offset in stabilization.c. Since the
same behavior can be achieved by leaving the "TrimAnglesSettings" at
0, there is no real reason to have two modes.

This makes the behavior of Attitude mode consistent with the old Plus
mode and removes AttitudePlus as an explicitly different mode.