Get rid of stored merkle branches.

Do not store merkle branches inside the wallet. This means removing the check
that someone might have inserted an unconfirmed transaction in your wallet and
making it look like a confirmed one.

This means we don't need to keep the merkle branch in wallets (which takes up to
half a kilobyte per transactions), we don't need to validate it (which requires
a dozen sha256 operation per wallet transaction), remove the need to keep the
merkle tree in blocks (the case where it's needed is done separately in
CPartialMerkleTree), and simplifies the merkle root computation.