Drop obsolete ExecJS.runtime attr_reader

This `attr_reader` is redifined right after the `module.rb` is evaluated
by `lib/execjs.rb`:

https://github.com/rails/execjs/blob/master/lib/execjs.rb#L5-L7

Author: viralpraxis

lib/execjs/module.rb

@@ -8,8 +8,6 @@ class ProgramError              < Error; end
   class RuntimeUnavailable < RuntimeError; end
 
   class << self
-    attr_reader :runtime
-
     def runtime=(runtime)
       raise RuntimeUnavailable, "#{runtime.name} is unavailable on this system" unless runtime.available?
       @runtime = runtime