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 5c38e42161)

.gitignore

@@ -132,23 +132,9 @@ cppcheck-cppcheck-build-dir/
 *.pydevproject
 *.launch
 
-# Gcov and Lcov code coverage
-*.gcno
+# GCOV code coverage
 *.gcda
-*.gcov.html
-*.func.html
-*.func-sort-c.html
-*index-sort-f.html
-*index-sort-l.html
-*index.html
-godot.info
-amber.png
-emerald.png
-glass.png
-ruby.png
-snow.png
-updown.png
-gcov.css
+*.gcno
 
 # Geany
 *.geany