app_confbridge: Fix error messages on exiting conference.

app_confbridge: Fix error messages on exiting conference.

A marked user ending a conference with only end_marked users generates
error messages:
ERROR[0000][C-00000000]: confbridge/conf_state.c:47 conf_invalid_event_fn: Invalid event for confbridge user ''

* The MULTI_MARKED state was doing too much when it was kicking out the
end_marked users from the conference.  The kicked out users will clean up
after themselves when they exit the conference.

(closes issue ASTERISK-20991)
Reported by: Jeremy Kister
Tested by: rmudgett
........

Merged revisions 380892 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/11.2@381415 65c4cc65-6c06-0410-ace0-fbb531ad65f3