Remove description method from abstract Linter

We don't want linter subclasses to be implementing this method as a
means of defining a default message, since it somewhat obfuscates where
the message comes from.

Remove it now that all references to `description` have been removed.

Change-Id: I0360927b3e72067136654d177980a4ee279fb8d0
Reviewed-on: http://gerrit.causes.com/36727
Tested-by: jenkins <[email protected]>
Reviewed-by: Shane da Silva <[email protected]>

lib/scss_lint/linter.rb

@@ -18,22 +18,16 @@ def run(engine, config)
       visit(engine.tree)
     end
 
-    # Define if you want a default message for your linter
-    # @return [String, nil]
-    def description
-      nil
-    end
-
     # Helper for creating lint from a parse tree node
     #
     # @param node_or_line [Sass::Script::Tree::Node, Sass::Engine::Line]
-    # @param message [String, nil]
-    def add_lint(node_or_line, message = nil)
+    # @param message [String]
+    def add_lint(node_or_line, message)
       line = node_or_line.respond_to?(:line) ? node_or_line.line : node_or_line
 
       @lints << Lint.new(engine.filename,
                          line,
-                         message || description)
+                         message)
     end
 
     # @param source_position [Sass::Source::Position]