xenserver: Do not set or remove vSwitchVersion xapi parameter anymore.

xenserver: Do not set or remove vSwitchVersion xapi parameter anymore.

The other-config:vSwitchVersion parameter was used to announce the Open
vSwitch version installed on a XenServer host, but this had the problem
that it could not be read or updated if the connection to the pool master
was down.  Because of this problem, the only user of this parameter in
Open vSwitch was removed (in commit 3cdc31a4c3a "xenserver: Retrieve
vSwitch version from binary in xsconsole").  So this commit finished the
process and removes the parameter entirely.

This should fix hangs on Open vSwitch installation and removal due to
waiting on the connection to the pool master.