Remove presence management

The feature is incredibly buggy and doesn't work as expected due to server behaviour and client interaction. One of the major problems is the constantly confused presence state - this is caused by the mobile apps conflicting on the state of the web app, causing it to consider the user offline or online (and rarely away) depending on how riot-android/ios is behaving at the time.

This reverts two PRs:
* https://github.com/matrix-org/matrix-react-sdk/pull/1620
* https://github.com/matrix-org/matrix-react-sdk/pull/1482

The changes to the context menu positioning were not reverted as they are useful outside of presence management.

Signed-off-by: Travis Ralston <travpc@gmail.com>

src/MatrixClientPeg.js

@@ -98,7 +98,6 @@ class MatrixClientPeg {
         const opts = utils.deepCopy(this.opts);
         // the react sdk doesn't work without this, so don't allow
         opts.pendingEventOrdering = "detached";
-        opts.disablePresence = true; // we do this manually
 
         try {
             const promise = this.matrixClient.store.startup();