They can't add new methods to any existing interfaces, or it would break all implementations by other developers. This actually is already the case with
JDBC. PreparedStatement, ResultSet and a few other interfaces have received extra methods in Java 6 that will not be implemented by older drivers. Perhaps the JDBC framework has solved this by catching errors and transforming them in SQLExceptions (e.g. SQLFeatureNotSupportedException), but it's also possible that you get a NoSuchMethodError instead (I haven't tested this yet). Perfect.
However, as Winston said, there is no need for a method like that.
remove accepts any object as its argument which mean that a
delete(E) method would do exactly the same but with some extra compiler checking. During runtime the two would be equivalent since type erasure would turn
delete(E) into
delete(Object).