Remove wifiConfiguration.isVisibleToUser()

This method seemed like a good idea at the time (quickly check whether a
network configuration is visible to a given user). However, once managed
profiles come into play, this simple method is no longer appropriate.
There is a more comprehensive replacement at
com.android.server.wifi.WifiConfigurationUtil.isVisibleToAnyProfile()
now and this method here can go.

BUG:26867426

Change-Id: Ia66aa6f74003a16eca24c5088d3aba8e0c9f8140