Remove unused accessor

Library/Homebrew/build_options.rb

@@ -7,7 +7,6 @@ class BuildOptions
 
   attr_accessor :args
   attr_accessor :universal
-  attr_accessor :cxx11
   attr_reader :options
   protected :options
 
@@ -104,7 +103,7 @@ class BuildOptions
 
   # True if the user requested to enable C++11 mode.
   def cxx11?
-    cxx11 || args.include?('--c++11') && has_option?('c++11')
+    args.include?('--c++11') && has_option?('c++11')
   end
 
   # Request a 32-bit only build.