Remove accidentally exposed individual methods from Class-File API

A few methods are exposed by Class-File API by accident: they see no use by any client, and their continued exposure will only be a maintenance in the long run. They are:
 - ModuleAttributeBuilder.build: this builder is already using the build(Consumer<Builder>) model, so the build method is useless.
 - CodeRelaber.relabel: internal access method, can just be hidden
 - ConstantPoolBuilder.writeBootstrapMethods: users shouldn't access the writing functionalities

These methods can be easily removed and should be removed.