mining: Remove unneeded disapproval check

[matheusd]

This removes a now unneeded check for unmineable transactions due to
block disapproval from the template generation code.

Previously this check was meant to ensure that the list of transactions
considered for inclusion in a block template did not include
transactions with an input referencing a regular transaction from a
parent disapproved block.

However this check is redudant due to check executed earlier in the
template building loop that asserts all inputs are valid utxos
according to the PoV of the parent block and taking into account whether
the new block being created will approve or disapprove it.

The check being removed had a bug due to the dcrutil.Tx being duplicated
without also copying the internal tree, which caused transactions to
later fail to be added to the template due to an unknown tree being
used.

Removing this corrects template generation for disapproved blocks. This can be verified by running the current code with --debuglevel=MINR=trace and configuring the wallet to disapprove some block. Notice there will be errors such as:

MINR: Error adding tx fc5371054e6f211bb7105ce30f60784c82089228b302865e6fb269d96893bf71 to block; invalid tree

[davecgh]

Nice observation. I confirmed this is indeed the case and tested that it works as expected with disapproved blocks.