Fix web-socket timeout closure exceptions

Fix web-socket timeout closure exceptions

As noted in WebSocketSessionHandler.onWebSocketClosed(), there is no
guarantee the session reports isOpen(). Performing the same check in
AbstractCommonSubscriber.removeSubscriber() (via isConnected()) can
therefore result in an ISE.

Fix this by removing the check.

JIRA: NETCONF-970
Change-Id: I6e437d9821e698e3faca121402a97b88ac486229
Signed-off-by: Ruslan Kashapov <ruslan.kashapov@pantheon.tech>
(cherry picked from commit 0d05f5be80762b833acc0b8337b68b353e4c673a)