[api] OBSEngine::Base#mount_it is not used

We never create an instance of `OBSEngine::Base`, so having and
instance method is pointless.

What we actually do is calling the class method with the same name in
all the subclasses. So, the `mount_it` method in `OBSEngine::Base`, but
I guess the idea was to leave the method also in the parent class as
help. In this case, this method should be the class method and not the
instance one.

src/api/lib/engines/base.rb

@@ -2,7 +2,7 @@ module OBSEngine
   # base class for all engine hooks
   class Base
     # implement this function to patch the routes
-    def mount_it
+    def self.mount_it
     end
   end