Initial pass over the source code to remove duplicated comments

The Jikes RVM source code currently contains many duplicated comments. This decreases both maintainablity and readability of the soure code. We should do the following:

• change the source level in the build to Java 6 (@Override for implementations of interface methods was added in Java 6 and might not be supported in all Java 5 compilers)
• add missing @Override annotations
• remove duplicate and near duplicate comments

See this thread for the original discussion.