Remove outdated entries from gitignore

.gitignore

@@ -84,13 +84,3 @@ moc_*.cxx
 *.[ao]
 *.so
 venv/
-
-## recastnavigation unused files
-extern/recastnavigation/.travis.yml
-extern/recastnavigation/CONTRIBUTING.md
-extern/recastnavigation/Docs/
-extern/recastnavigation/Doxyfile
-extern/recastnavigation/README.md
-extern/recastnavigation/RecastDemo/
-extern/recastnavigation/Tests/
-extern/recastnavigation/appveyor.yml