cpu: remove unnecessary data ptr from O3 internal read() funcs

cpu: remove unnecessary data ptr from O3 internal read() funcs

The read() function merely initiates a memory read operation; the
data doesn't arrive until the access completes and a response packet
is received from the memory system.  Thus there's no need to provide
a data pointer; its existence is historical.

Getting this pointer out of this internal o3 interface sets the
stage for similar cleanup in the ExecContext interface.  Also
found that we were pointlessly setting the contents at this pointer
on a store forward (the useful memcpy happens just a few lines
below the deleted one).