Remove FalseClass#to_i core extension.

Remove a monkey patch that added a `to_i` method to `FalseClass` so that
`false.to_i` returned 0. This is legacy code that shouldn't still be in
use anywhere. It doesn't really work anyway, because `true.to_i` isn't
defined.

config/initializers/core_extensions.rb

@@ -62,9 +62,3 @@ end
 class String
   include Danbooru::Extensions::String
 end
-
-class FalseClass
-  def to_i
-    0
-  end
-end