[FEATURE] Trustlines: Add ability to delete trust line (WC-1620)

Checks for 3 cases:

1) Trustline has zero balance -> Delete trustline
2) Trustline has balance & there is a market to convert remaining balance to XRP -> Convert remaining balance to XRP to send back to user, delete trust line
3) Trustline has balance & there is no market to convert remaining balance -> Send remaining balance back to the issuer, delete trustline