Quantity remove equivalencies

Following the discussion on astropy-dev [1], this PR removes the option to give `equivalencies` to `Quantity` objects. It keeps the implementation, though, so they can be used for `Quantity` subclasses (such as `Magnitude`; see #1577). For individual objects, `equivalencies` should either be passed explicitly to the `.to` method, or set globally/per context (see #1593).

Note: Equivalencies were also removed from `Angle`. Furthermore, test cases were changed to test having a class-based `equivalencies` set.

[1] https://groups.google.com/forum/#!topic/astropy-dev/jihIn4n59vE