Remove aint8 type

The aint8 type represented an asymmetrically quantized range.
With the change to move scaling fully into the RESCALE operator,
the aint8 and int8 types didn't have significant differences in
their use with operators. Unifying to a single data type makes
things simpler.

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: I3ee89cbdafdc61293ef1f1bec52398d744e716a1