blockchain: remove impossible to hit ErrInvalidSSRtxInput.

[dnldd]

ErrInvalidSSRtxInput's triggering condition is a subset of the checks IsSSRtx performs on a transaction. Since IsSSRtx is the first to evaluate a tx in CheckTransactionInputs that should be enough.

This is work towards #1182

[davecgh]

I'm closing this because it is not correct for a couple of reasons.

First, the description provided in the PR is incorrect because the check is ensuring the the previous output that is being referenced by the revocation is a ticket, while IsSSRtx only deals with the transaction itself, which is evidenced by the fact the function does not accept a UtxoViewpoint. That means it is impossible for IsSSRtx to perform this check.

Second, while it is true that the error shouldn't be possible to hit from a block, due to the fact that an earlier check will fail on the referenced output not being in the set of missed tickets, removing this would incorrectly allow the mempool to accept revocations that don't actually reference tickets since it relies on the CheckTransactionInputs function to detect this condition and return an error accordingly.

Given that, if these checks were to be removed, they would somehow need to be separated and independently checked from mempool.