MAM: Simplify _parse_chat_attrs()

This became more simple due to nbxmpp changes
3 jobs for master in 2 minutes and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #13556
run-appdata

00:00:05

passed #13555
run-tests

00:02:28

 
  Build
passed #13557
run-build

00:00:09