[Cashtab] Remove unused validation function

Summary:
In D15884, we tightened validation for contact list (only accept ecash: prefixed addresses). So, we stopped using this function. If we start accepting aliases, we need to convert them to addresses before storing them to keep app functionality straightforward.

For now, this is not used anywhere, so get rid of it.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15919