Remove dead _sendPingToDevice function
Remove dead _sendPingToDevice function

This is no longer used; remove it.

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