Remove zp subtraction from tensor_read pseudocode

Operators which use the zero-point functionalty for 8-bit integer
processing are updated to do the zero-point subtract in their
pseudocode.

Note that the PAD operator no longer takes a zero point argument,
and instead requires callers to account for the zero point in the
pad_const argument.

Change-Id: I3bca1cae85aa2093000c420f0433633c347a29de