doc/REST-interface: Remove stale info

Summary: This is a backport of core#19390

Test Plan:

src/bitcoind -rest
curl localhost:8332/rest/getutxos/checkmempool/8818bd451fd3caf8b0e36ea4f49429a3690bfc6900729c2bc059c071dc37eaf5-0.json

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9964