Remove native bind guard.

Previous discussion on this topic is in ce3d1aec306999aa94926a42cad1daf7eb87a36f.

If we're going to drop support for certain features of `_.bind` in browsers that do not support `Function#bind`, I think we should do so across the board.  The features in question (e.g. binding constructor functions) are not nearly as important as supporting or not supporting them consistently.