kernel: Eliminate unused memory pool structure field

Eliminates the memory pool field indicating the size of the
pool's memory buffer, since it isn't used anywhere.
(Anyway, it could be computed by multiplying the maximum
block size by the number of maximum-sized blocks.)

Change-Id: Ia11554bdc2b246a1ba0ea33f05c5e6ce6a32ca13
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>