Un-mixin ReactBrowserComponentMixin from ReactTextComponent

ReactTextComponent having `getDOMNode` is an implementation detail, it's also very likely that it won't support it in the future (and since it's technically not always a span, it's even wrong in some sense today).

PS. Can you even access it today without cheating your way in and using private members? I don't think so...
Lint complains that `textNode` is already defined, want to fix that?