Remove rules argument from getblocktemplate help

Summary: This was mistakenly added in D5548.

Test Plan:

ninja
./bitcoind
./bitcoin-cli help getblocktemplate

Verify the change in help text.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5572