Remove unnecessary check on __set__ parameters.

This commit removes parameter checks on the `instance` argument to the `__set__` methods for descriptors. These checks are not necessary - the [documentation on descriptors](http://docs.python.org/release/2.7.4/reference/datamodel.html#object.__set__) makes it clear that the `__set__` methods will always be called with a non-null `instance` parameter (unless you're doing something silly like `MyClass.__dict__['mydesc'].__set__(None, None)` of course) More info available via [this question on stackoverflow](http://stackoverflow.com/questions/16412672/can-a-python-descriptors-set-ever-get-called-with-instance-none).

I'm guessing this doesn't need a ticket, but if it does, let me know and I'll open one.