util: Fail to parse empty string in ParseMoney

Summary:
Remove unused ParseMoney that takes a c_str, fail to parse empty string

Supplying a fee rate or an amount on the command line as an empty string, which currently parses as 0 seems fragile and confusing.

This is a backport of Core PR18225

Test Plan: ninja && ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8775