Cleanup remaining instances of txconfirmtarget

Summary: Confirm target is no longer used, so cleaning it up. This operation is safe because now that txconfirmtarget will be simply ignored.

Test Plan:

make check
bitcoin-qt # for sanity
bitcoin-qt --help # no longer contains help message for -txconftarget

Added txconfirmtarget=2 to bitcoin.conf
bitcoind operation was not disrupted.

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2672