Revert "Call MatrixClient.clearStores on logout"
This reverts commit c3d37c1ff9f3cb30834bb6e97c49845be9363b2f.
This commit was introducing a bug where no rooms would be shown if
you want straight to /#/login and logged in (because this causes
a guest session to be created but the indexeddb store not to be
cleared, so the new login picks up the stale indexedb sync data.
