Base64: Strip out code JGit doesn't use

Base64: Strip out code JGit doesn't use

Since we have already modified this class to localize an error
message, we might as well strip it down to contain only the
functionality we need, or might ever use.

To keep this simple to review we don't adjust formatting right
away, so code that was buried inside of an if or else block whose
condition was removed might not have the correct indentation anymore.
We can fix this with a later reformatting change.

Change-Id: I2996aaa704e9d6182e5500c7a63240d5e9d722cc
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>