These NetconfClientSessionPreferences are a strictly netconf-client thing,
remove it and adjust its users. This also renders
NetconfSessionPreferences useless, hence those are removed as well.
JIRA: NETCONF-590
Change-Id: I72c5718182751cb869c13318b4b0133bdbd197ea Signed-off-by: Robert Varga <robert.varga@pantheon.tech>