connector: remove unnecessary config key

[frsyuki]

connector commands (like connector:guess) requires config key. But it is unnecessary.
On the other hand, embulk will add multi-job feature (https://github.com/embulk/embulk/issues # 167). Once it is implemented to embulk, configuration file will be more complex. In this case, if both embulk and td CLI use the same config syntax, we can use the same document for users (although it is not the best solution, it's good for customer support if there is some documents at embulk.org). If only td commands need config key, we can't use the same document.

[frsyuki]

@nahi related to #67.