Drop asm-util from the build

The only reference to asm-util in the build is a string constant in `DebuggingClassWriter` where it looks up the tracing class visitor by reflection (to handle situations when asm-util may not be on the runtime classpath):

https://github.com/cglib/cglib/blob/master/cglib/src/main/java/net/sf/cglib/core/DebuggingClassWriter.java#L41

Alternatively the asm-util dependency could be made optional in the parent pom.