Remove useless removed-tx tracking from mempool remove calls

Built on top of #5267.

To see why this clearly is redundant, take the most simple case: a tx enters the mempool, we call SyncWithWallets(tx, NULL). It is then overridden by a conflicting transaction in a block, we call SyncWithWallets(tx, NULL); SyncWithWallets(conflictor, newblock);.
I tested that transactions are still getting marked conflicted by the conflictor SyncWithWallets (which is where it should happen), and @gavinandresen said some test he had previously set up was still passing (dont recall exactly what), though @dgenr8 claimed he was not seeing the same. Maybe @dgenr8 could clarify the reproduction steps?