Fix Graph deletion which cause NPE

Fix Graph deletion which cause NPE

Graph is deleted twice when stopping ODL which cause a NPE.

When ODL stops, AbstractTopologyBuilder() calls
LinkStateGraphBuilder.clearTopology() which call
ConnectedGraphImpl.clear(). Then ConnectedGraphServer() stops by
calling destroyOperationalGraphModel() which browse all stored graphs
and call again ConnectedGraphImpl.clear() which cause the NPE.

Fix removes graph from ConnectedGraphServer graphs Map in
clearGraph() method instead of deleteGraph() method.

JIRA: BGPCEP-977

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>
Change-Id: I55209e77ffa2c317ea85ca29cc794016f338c488