Remove .gitattributes file (#14676)

I added the `.gitattributes` a while back because I thought it would
help us make it simpler when rebasing changes to the `CHANGELOG.md`
file. However, I don't think this is actually true. Now, instead of a
merge conflict, the rebase just ends up duplicating a lot of lines which
we only see when we review the PR on GitHub later which is really
annoying.

So, for now, let's remove it again. Ideally we can have a better merge
strategy here (or potentially other tools).

Author: philipp-spiess