DCM: drop the 'drop z' method

the 'drop z' method reduced the impact of noise on omegaI, but it also
made us more sensitive to errors in accelerometer calibration and
scaling, as demonstated by the logs from Gabor here:

  http://diydrones.com/xn/detail/705844:Comment:834373

Simulation testing shows that the other noise suppression methods
applied in the DCM code, in particular the slope limiting on omegaI
the removal of the weighting and the upcoming use of a _omega_I_sum
buffer have reduced the impact of noise enough that we can now safely
include z in the acceleration calculation.