remove unused functions from libnet

The linktime section-gc found a couple of other potentially unused functions / variables in libnet ;
  example output of --print-gc-sections for the libnet product - build (linux s390x) :

/bin/ld: Removing unused section '.bss.my_gconf_init_func' in file '/builddir/support/native/java.base/libnet/DefaultProxySelector.o' <--- seems to be dead
/bin/ld: Removing unused section '.text.getInet6Address_scopeifname' in file '/builddir/support/native/java.base/libnet/net_util.o' <--- seems to be dead
/bin/ld: Removing unused section '.text.getInet6Address_scopeid_set' in file '/builddir/support/native/java.base/libnet/net_util.o' <--- seems to be dead
/bin/ld: Removing unused section '.text.getInetAddress_hostName' in file '/builddir/support/native/java.base/libnet/net_util.o' <--- seems to be dead
/bin/ld: Removing unused section '.text.setDefaultScopeID' in file '/builddir/support/native/java.base/libnet/net_util_md.o' <--- seems to be dead indeed
/bin/ld: Removing unused section '.text.getDefaultScopeID' in file '/builddir/support/native/java.base/libnet/net_util_md.o' <--- seems to be dead indeed
/bin/ld: Removing unused section '.text.kernelIsV24' in file '/builddir/support/native/java.base/libnet/net_util_md.o' <--- seems to be dead indeed
/bin/ld: Removing unused section '.bss.ni_defaultIndexID.8722' in file '/builddir/support/native/java.base/libnet/net_util_md.o' <--- only used in getDefaultScopeID , which is dead
/bin/ld: Removing unused section '.bss.ni_class.8721' in file '/builddir/support/native/java.base/libnet/net_util_md.o' <--- only used in getDefaultScopeID , which is dead
/bin/ld: Removing unused section '.bss.vinit24' in file '/builddir/support/native/java.base/libnet/net_util_md.o' <--- only used in kernelIsV24 which is dead
/bin/ld: Removing unused section '.bss.kernelV24' in file '/builddir/support/native/java.base/libnet/net_util_md.o' <--- only used in kernelIsV24 which is dead

Those can be cleaned up.