Replace usage of our Base64 implementation with java.util.Base64

Class org.apache.hadoop.hbase.util.Base64 has been removed in it's entirety from HBase 2+. In HBase 1, unused methods have been removed from the class and the audience was changed from Public to Private. This class was originally intended as an internal utility class that could be used externally but thinking since changed; these classes should not have been advertised as public to end-users. This represents an incompatible change for users who relied on this implementation. An alternative implementation for affected clients is available at java.util.Base64 when using Java 8 or newer; be aware, it may encode/decode differently. For clients seeking to restore this specific implementation, it is available in the public domain for download at http://iharder.sourceforge.net/current/java/base64/