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).

(cherry picked from commit 5c38e4216174f21362058a1a77938f5a63e2ffef)

.gitignore

@@ -69,6 +69,10 @@ gmon.out
 *.pydevproject
 *.launch
 
+# GCOV code coverage
+*.gcno
+*.gcda
+
 # Geany/geany-plugins files
 *.geany
 .geanyprj