[Cashtab] Delete function that is handled better by lib

Summary:
Cashtab gets the hash160 by calling the old cashaddr library version, then converting to a legacy address to then use a different lib to extract the hash160.

We don't need to do it this way anymore, ecashaddrjs will just return it as a string.

Delete the old function and remove its associated dependency. Replace with equivalent lib method.

Test Plan:
npm test
grep -r toHash160 src/ and no output
grep -r bs58 src/ and no output

Reviewers: #bitcoin_abc, Fabien, PiRK

Reviewed By: #bitcoin_abc, Fabien, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15273