Merge #11027: [RPC] Only return hex field once in getrawtransaction

Summary:
6bbdafc Pass serialization flags and whether to include hex to TxToUniv (Andrew Chow)
e029c6e Only return hex field once in getrawtransaction (Andrew Chow)

Pull request description:

The hex is already returned in `TxToUniv()`, no need to give it out a second time in getrawtransaction itself.

Tree-SHA512: 270289f2d6dea37f51f5a42db3dae5debdbe83c6b504fccfd3391588da986ed474592c6655d522dc51022d4b08fa90ed1ebb249afe036309f95adfe3652cb262

Backport of Core PR 11027
https://github.com/bitcoin/bitcoin/pull/11027
Completes T564

Test Plan:
ninja check
test_runner.py

Run bitcoin-cli <txid> 1 and ensure that there is a single top-level "hex" field in the returned JSON.

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2709