Edit ext-tx so it isn't doing redundant prunes

The original pruning function was not taking into account
that certain tx sizes/block sizes use a reduced tx set.

Prune 1: -0.3% performance drop, 20% speedup on foreman video
Prune 2: -0.48% perfomance drop, 30% speedup on foreman video

Change-Id: I557e919d97a89f787b47b3c8579a080db57f91d0