Drop code coverage bot config.

I have little idea what this did,
and found its visualizations more confusing than useful.

I also have little fondness for cloud-based services of this kind
if they don't have any useful contribution to localhost development,
and I think we can easily look at this file and see that it said
nothing of use.

Per the advice of https://github.com/ipld/go-ipld-prime/pull/49#issuecomment-595552836 ...

RIP IT OUT

codecov.yml

-coverage:
-  precision: 2
-  round: up
-  range: "50...90"
-  status:
-    project: off
-    patch: off
-