Fully remove codecov
Fully removes codecov as we don't have any full test suite in circleci, alongside https://github.com/huggingface/transformers/commit/829b9f8cc321aa28396e6203e0f21eed26b132f7

If we want to put it back up, we can merge the two slow tests (TF + PT) and run coverage on that, but we should first take care of the inconsistencies in coverage as explained in https://github.com/huggingface/transformers/issues/6317

cc @stas00 
If we try to go back to using it, perhaps there is a way to merge reports from 2 half-full tests - otherwise it creates an unnecessary slowdown on CI  to run both together if we don't need to.