ref(NetworkUtils): Remove unused utils.

[bgrozev]

These utilities are no longer used by jitsi desktop or any other jitsi project I could find, so imo they are not worth maintaining in ice4j:

boolean isValidIPAddress(String address)
boolean isIPv4Address(String address)
boolean isIPv6Address(String address)
byte[] strToIPv4(String ipv4AddrStr)
byte[] strToIPv6(String ipv6AddrStr)
byte[] mappedIPv4ToRealIPv4(byte[] addr)

There are readily available alternatives in e.g. Guava's InetAddresses:

boolean isInetAddress(String ipString)
byte[] ipStringToBytes(String ipStringParam)
Inet4Address getEmbeddedIPv4ClientAddress(Inet6Address ip)