Drop the dependency commons-codec for Base64

Current Result

https://github.com/zkoss/zk/blob/v9.0.0/zweb/src/org/zkoss/web/fn/ThemeFns.java#L203-L205
in zweb we use commons-codec just for Base64.

Expected Result

https://docs.oracle.com/javase/8/docs/api/java/util/Base64.html
Use built-in java.util.Base64 instead.

And remove the commons-codec dependency.