Revert "Handle Harmonia task re-assignment."

This reverts commit de8fc182c6de70ffad791f806ef5f2a781a62efd.

I saw the following exception in production on task assignment:

    Encoding::InvalidByteSequenceError - "\xE2" on US-ASCII

    /usr/lib/ruby/2.1.0/json/common.rb:155:in `encode'
    /usr/lib/ruby/2.1.0/json/common.rb:155:in `initialize'
    /usr/lib/ruby/2.1.0/json/common.rb:155:in `new'
    /usr/lib/ruby/2.1.0/json/common.rb:155:in `parse'
    /home/webhooks/app/vendor/gems/ruby/2.1.0/gems/ruby-trello-1.1.1/lib/trello/core_ext/string.rb:19:in `json_into'
    /home/webhooks/app/vendor/gems/ruby/2.1.0/gems/ruby-trello-1.1.1/lib/trello/basic_data.rb:38:in `parse_many'
    /home/webhooks/app/vendor/gems/ruby/2.1.0/gems/ruby-trello-1.1.1/lib/trello/client.rb:54:in `find_many'
    /home/webhooks/app/vendor/gems/ruby/2.1.0/gems/ruby-trello-1.1.1/lib/trello/basic_data.rb:95:in `block (2 levels) in many'
    /home/webhooks/app/app.rb:41:in `block in <top (required)>'

This seems to be related to the query for all the cards in a list which was
added in this commit.