[ahrs] remove AHRS_UPDATE_FW_ESTIMATOR and ins_x_neutrals
[conf] remove ins_neutrals settings from example and test conf
[modules] hackhd: remove ins neutral
[control] add a pitch trim to replace the ins_pitch_neutral
Merge pull request #816 from paparazzi/rm_ins_neutrals

remove AHRS_UPDATE_FW_ESTIMATOR and ins_x_neutrals