Deletion of trust lines

[ckeshava]

Trust lines that have default settings need to be deleted from an account's directory. I'm not using any flags on either end accounts of the trust line.

This tip of this branch has unit tests to that effect: https://github.com/XRPLF/xrpl.js/pull/new/updateREADME

I didn't observe the expected behavior, but I'm yet to ascertain the cause of this issue. Similar unit test on rippled codebase appears to be working correctly: XRPLF/rippled#4886. Hence, I'm inclined to check for discrepancies in the javascript implementation.

This thread has relevant discussion on this issue: XRPLF/rippled#4698 (comment)