Don't expose 'connected' as a promise.

'reconnected' cannot be used as a promise (because it's recurring),
so to avoid confusion let's not have a 'connected' promise either.

docs/source/developer_api.rst

@@ -912,7 +912,6 @@ The core events, which are also promises are:
 
 * cachedRoster
 * chatBoxesFetched
-* connected
 * pluginsInitialized
 * roster
 * rosterContactsFetched

src/converse-core.js

@@ -155,7 +155,6 @@
     const PROMISES = [
         'cachedRoster',
         'chatBoxesFetched',
-        'connected',
         'pluginsInitialized',
         'roster',
         'rosterContactsFetched',