[cpp client] - Remove KuduSession::SetPriority()
[cpp client] - Remove KuduSession::SetPriority()

There was a stub declaration on the header for this, but no implementation
existed, so the method was removed.

Change-Id: I738635a9a7a15b8437b197a48adab01a9bcd9a3c
Reviewed-on: http://gerrit.cloudera.org:8080/1424
Reviewed-by: David Ribeiro Alves <33ea948168c114d220e0372a903be6ee60f6396e@cloudera.com>
Tested-by: Internal Jenkins

andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,cloudera/kudu