Temporarily disable project coverage

[wilzbach]

At CircleCi we merge the PR into the respective Phobos branch (e.g. master, stable, ...) to simulate its effect on the regarding branch and to auto-update the circleci.sh and posix.mak.
However, CodeCov doesn't these merges very well. In particular they completely ignore the new parent_ref and assume it to be the one of the PR. This means that all commits between the parent_ref of a PR and the current upstream/master are also counted in the codecov/project information output.
Hence, it usually looks pretty random and often shows red warnings.

-> a solution is to remove the codecov/project report until CodeCov can handle parent_ref properly. The information on the coverage changes introduced by a PR was and will stay correct, except for CTFE and DMD bugs.
To the people using the CodeCov extension to look at PRs: it's worth noting that due the incorrect handling of parent_ref CodeCov the coverage report might also be shifted by X lines (as there might have been a commit to the same file between the parent_ref of the respective PR and upstream/master)

Learn more:

https://github.com/codecov/support/issues/360

[wilzbach]

See also an alternative at #5198

Instead of disabling CodeCov, we simply revert the merged source code changes, but keep the updated circleci.sh and posix.mak file.

[wilzbach]

So it turns out that CodeCov was behaving perfectly well, but Phobos testsuite has a lot of random elements in it that keep being (un)-covered :/

For more details:
#5202

(sorry for the noise)