Remove unneeded nil checks

The library we're using, default_value_for, is configured to interpret nil
as the default value, being ENABLED. Given the property can't be nil
this check is unneeded.

app/models/project_feature.rb

@@ -49,20 +49,14 @@ class ProjectFeature < ActiveRecord::Base
   end
 
   def builds_enabled?
-    return true unless builds_access_level
-
     builds_access_level > DISABLED
   end
 
   def wiki_enabled?
-    return true unless wiki_access_level
-
     wiki_access_level > DISABLED
   end
 
   def merge_requests_enabled?
-    return true unless merge_requests_access_level
-
     merge_requests_access_level > DISABLED
   end