Remove now-retired package.json property
Remove now-retired package.json property

For https://github.com/matrix-org/matrix-react-sdk/pull/3723
vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/riot-web