Revert "[AE/CA] - fix the matrix mixer by obeying a setpwidth of the number of output device channels (should fix mixing issues where user reported stereo files appear mono only and maybe some other strange things)"

This reverts commit 6353323f3e834d95fb17bc35794645ba1060c963.

The mixing unit is not the same as device. The output's in the mixing
unit doesn't match the device output. So the matrix it has internally
doesn't have anything to do with device outputs.
CoreAE: validate that matrix fit's in mixer size
CoreAE: make sure we zero out any left over output channels during mixing