Remove IsValidDestinationString overload that doesn't use a Config parameter.

Summary: This is one more step toward getting rid of the config as a global parameter.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1505