java-utils-2.eclass: Call eapply_user and drop java_prepare for EAPI 6

java-utils-2.eclass: Call eapply_user and drop java_prepare for EAPI 6

Although simply calling default sounds like a good idea, if an ebuild
needs to use multiple eclasses that call default then patching will
break as PATCHES will be applied more than once. eapply_user, on the
other hand, is idempotent. If an ebuild needs to apply PATCHES then it
should define src_prepare and call default explicitly. This will
probably be improved in EAPI 7.

java_prepare is being dropped because non-Java people find it
confusing and it isn't consistent. Other eclasses don't do this and we
don't apply the same rule to other phases either.