[Sass] Get rid of some now-useless interpolation code in Script::String.

lib/sass/script/string.rb

@@ -2,15 +2,6 @@ require 'sass/script/literal'
 
 module Sass::Script
   class String < Literal # :nodoc:
-    INTERPOLATION = /(^|[^\\])\#\{([^}]*)\}/
-    #TODO pass line & char context to perform
-    def perform(environment)
-      interpolated = @value.gsub(INTERPOLATION) do |match|
-        "#{$1}#{Sass::Script.resolve($2, 0, 0, environment)}"
-      end
-      Sass::Script::String.new(interpolated)
-    end
-
     def to_s
       @value
     end