Remove non-existent paths from codecov-ignore

[brownbaerchen]

Presumably, these paths have existed at some point, but they don't anymore and we should remove this dead code.

$ ls heat/core/regression/
ls: heat/core/regression/: No such file or directory
$ ls heat/examples/
ls: heat/examples/: No such file or directory
$ ls heat/**/test_*.py
zsh: no matches found: heat/**/test_*.py