Remove assertions for redundant SerializerMethodField source args.

[gtaylor]

As discussed in #2420, removed SerializerMethodField's assertion raised when redundant source params are passed.

It looks like this was also in the base Field class. For the sake of consistency, I removed this, too. While I think an error could make sense for something like a CharField on a ModelSerializer, it would be a bit of an inconsistent bit of policing. It's probably best left to the people using DRF to police their own codebases. FWIW, I'd definitely pick on redundant CharFields in my code reviews, but SerializerMethodFields are a lot more open-ended by definition.

If you wanted to take the split approach (policing for non-SerializerMethodField), I could add an optional kwarg to bind and have SerializerMethodField's init pass along a value to disable the policing. Though, that could gunk some people up since we'd be changing the signature of a pretty base level class.

The 3.0 release announcement were updated. I thought about adding an entry to the changelog under 3.0.4, but since I don't see a header for 3.0.4 yet I am assuming this is done as part of the release process.

All tests passed on tox, aside from the django-master stuff that was already failing beforehand.

[tomchristie]

We definatly do want to keep this check. This style is simply unneccessary and confusing.

This:

foo = fields.IntegerField(source='foo')

Is absolutely poor style, and we shouldn't allow it.

[tomchristie]

I'm willing to consider this for SerializerMethodField, but certainly not for source.