Drop hdfs functions from Ibis top-level module

[datapythonista]

In #2497, ibis.HDFS, ibis.hdfs_connect... have been moved to the Impala backend where they belong, and started to raise a FutureWarning if called directly from the top ibis module.

After releasing (maybe more than once, not sure what will make sense), we should drop them them complete from ibis and leave them only in ibis.impala.

[datapythonista]

@jreback ok to remove those in the next version? Since we're already showing a FutureWarning and we are planning to release a major version next (2.0), I think this is reasonable.

Removing them will make my life easier and the code cleaner with the entrypoints stuff.