Add API to delete threepid
Add API to delete threepid

Uses https://github.com/matrix-org/synapse/pull/1714

williamboman/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,williamboman/matrix-js-sdk,williamboman/matrix-js-sdk