Fix error when loading the install category (#167)

The removed `install` ref was causing an error when loading the `install` category. The ref used to exist back in 97daa3b, but the corresponding tutorial was removed in #157.

Author: azeey

manifest.xml

@@ -1080,9 +1080,6 @@
         <tutorial>install_on_mac</tutorial>
         <tutorial>install_on_windows</tutorial>
 
-        <!-- keep name for historical reasons -->
-        <tutorial>install</tutorial>
-
         <tutorial>install_from_source</tutorial>
         <tutorial>install_dependencies_from_source</tutorial>
         <tutorial>install_unstable</tutorial>