Retire libwebhdfs

This library is basically a mess:

• It's not part of the mvn package
• It's missing functionality and barely maintained
• It's not in the precommit runs so doesn't get exercised regularly
• It's not part of the unit tests (at least, that I can see)
• It isn't documented in any official documentation

But most importantly:

• It fails at it's primary mission of being pure C (HDFS-3917 is STILL open)

Let's cut our losses and just remove it.