remove codecov PR comments


#### Problem
+ @stas00 has tried very hard to get codecov working to no avail in https://github.com/huggingface/transformers/issues/6317
+ Files that are not affected by a PR show changes in coverage.
+ Codecoverage information is rarely/never useful
+ lots of distracting spam emails
+ lots of vertical space that could otherwise be used for reading discussion history.

#### Proposed solution:
The idea of codecov -- to warn people before they introduce untested code -- is good, but the current implementation is worse than nothing, and after significant effort (mostly from @stas00) I think it is time to give up. If we see another tool we like, or manage to reconfigure this one to work well, that's great, but I think that should happen without broken codecov on master.


The page is nice but the data seems wrong -- 
https://codecov.io/gh/huggingface/transformers/src/master/src/transformers/modeling_layoutlm.py
says that `__init__` is not covered, but I checked and it is.. in `test_modeling_layoutlm.py`.

I can also just disable PR comments if that works better for you.

<|||||>> Is there another tool we could leverage to give us more reliable information on our code coverage?

The tool is not the problem, the problem lies in our test suite being not idempotent. codecov only compares the old coverage to new coverage. It can't give correct coverage if the data it works with is invalid. Garbage in garbage out.

If you want an approximate coverage, you can just add  `cov: pytest --cov` in Makefile. It has a bunch of formats if you want the report in a particular format. It should be within 98% of correctness based on the current state of the test suite. <|||||>I understand the issue. Could we simply disable the PR comments for now @sshleifer, as that's the only pain point?<|||||>Done. Will merge once checks pass!