Remove DM-guessing code (again)
Seems this got reverted somehow:
>the revert of https://github.com/matrix-org/matrix-react-sdk/pull/807 (https://github.com/matrix-org/matrix-react-sdk/commit/ebfafb363972e77ef02f0e518573bf68ebc0ed15) also included the revert of https://github.com/matrix-org/matrix-react-sdk/pull/829
