Removed unused confirmation target dropdown from the UI

Summary: Confirmation targets do not affect tx fees, so having this on the UI is useless at best and confusing at worst. I'm also removing confirmation targets from txmempool::estimateFee's API in D2614, so there is no plan to use this in the future.

Test Plan:
ninja check
test_runner.py
bitcoin-qt

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2641