xds testing: delete test new_instance_group
xds testing: delete test new_instance_group

This test is already covered by remove_instance_group (which adds a new instance group and then remove).
This test is also failing when the TD instance connected by the client is in a different region.

stanley-cheung/grpc,nicolasnoble/grpc,stanley-cheung/grpc,ctiller/grpc,jtattermusch/grpc,stanley-cheung/grpc,ctiller/grpc,vjpai/grpc,grpc/grpc,donnadionne/grpc,nicolasnoble/grpc,ctiller/grpc,ctiller/grpc,vjpai/grpc,donnadionne/grpc,ejona86/grpc,ctiller/grpc,vjpai/grpc,donnadionne/grpc,donnadionne/grpc,stanley-cheung/grpc,ctiller/grpc,jtattermusch/grpc,jtattermusch/grpc,ctiller/grpc,donnadionne/grpc,grpc/grpc,grpc/grpc,vjpai/grpc,nicolasnoble/grpc,nicolasnoble/grpc,nicolasnoble/grpc,grpc/grpc,grpc/grpc,jtattermusch/grpc,ctiller/grpc,nicolasnoble/grpc,jtattermusch/grpc,ejona86/grpc,donnadionne/grpc,nicolasnoble/grpc,stanley-cheung/grpc,ctiller/grpc,jtattermusch/grpc,jtattermusch/grpc,nicolasnoble/grpc,ejona86/grpc,vjpai/grpc,stanley-cheung/grpc,donnadionne/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,nicolasnoble/grpc,ejona86/grpc,vjpai/grpc,stanley-cheung/grpc,grpc/grpc,jtattermusch/grpc,vjpai/grpc,stanley-cheung/grpc,grpc/grpc,ejona86/grpc,jtattermusch/grpc,ctiller/grpc,donnadionne/grpc,grpc/grpc,ejona86/grpc,ejona86/grpc,donnadionne/grpc,ejona86/grpc,donnadionne/grpc,vjpai/grpc,vjpai/grpc,ejona86/grpc,vjpai/grpc,ctiller/grpc,stanley-cheung/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,vjpai/grpc,nicolasnoble/grpc,ejona86/grpc,ejona86/grpc,grpc/grpc,vjpai/grpc,jtattermusch/grpc,ctiller/grpc,grpc/grpc,donnadionne/grpc,nicolasnoble/grpc,donnadionne/grpc,grpc/grpc,nicolasnoble/grpc,ejona86/grpc