DEV: turn off codecov inline annotations

[lucascolley]

There are false positives for missing coverage due to some coverage only being provided in the job which checks various backends. Codecov correctly combines reports and doesn't show these false positives for the online interface and in the CI statuses, but doesn't remove the annotations.

The value of the annotations in addition to the CI status seems very minimal, so probably best to just disable them.

[lucascolley]

indeed this is a documented limitation

[lucascolley]

hopefully that does it, can investigate further if not