delete.rb should require user to provide the column

HBASE-18142 made an intentionally incompatible change to delete.rb

Most notably from a user, the help message is now inaccurate: only two arguments must be provided (table name and row) whereas it was required to also provide a column to delete.

chunhao, chia7712, might one of you folks be able to submit a patch to fix this up (maybe get those release notes added to HBASE-18142 too? )