Replace plain with system-default in Socket and ServerSocket

Prior to JDK-8221481, there was just a single default SocketImpl, the
"plain" socket impl. There are now two, PlainSocketImpl and
NioSocketImpl, the latter being the default. The former can be selected
as the default by setting the `jdk.net.usePlainSocketImpl` system
property, further information an be found in the Implementation
Note of` java.net.SocketImpl`.

The term "plain" is mentioned in a few places in the specification of
j.n.Socket and j.n.ServerSocket. This should be updated to
"system-default", which is the agreed term post JDK-8221481.