Remove three auxilary files in tools/javac/enum dir

As part of the fix for JDK-8046618, following three files need to be deleted:
test/tools/javac/enum/T5081785a.java
test/tools/javac/enum/T5081785b.java
test/tools/javac/enum/T5081785c.java

The test cases in the above three files have been combined and pushed in test/tools/javac/enum/T5081785.java