Remove responseInterceptors collection from $httpProvider for 1.3

[caitp]

To be honest, I didn't even know we had a responseInterceptors collection to begin with. I only found out this existed because of a question on IRC.

But basically, it's super awkward to have 2 apis which essentially do an equivalent thing, with one being more obvious in the docs than the other. And getting rid of the responseInterceptors collection means there are a few lines of code and docs that can be removed, so this is a good thing.

I think it's worth dropping it for 1.3, in the name of clean API and simplicity.