Remove too greedy GCOV ignores from `.gitignore`
This would cause `updown.png` to be ignored in our default theme in 3.x.
These ignores were added in #36800 for #36572 (see that PR for usage
instructions).
From a quick test, using `--output-file` for `lcov` and `--output-directory`
for genhtml let us output the files in a way that won't conflict with the
Git repository (e.g. in `bin/`, or outside the Git repo).
Rémi Verschelde