ISCSIPlugin : remove specific volume_size_info

no need to use volume_size_info, we can use generic volume_size_info from Plugin. (file_size_info)

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>

PVE/Storage/ISCSIPlugin.pm

@@ -378,10 +378,5 @@ sub check_connection {
     return iscsi_test_portal($portal);
 }
 
-sub volume_size_info {
-    my ($class, $scfg, $storeid, $volname, $timeout) = @_;
-
-    return undef;
-}
 
 1;