Remove extension validation from parser

The check nesting visitor is responsible for AST validation. This
was left over from #2062.

Author: xzyfer

src/parser.cpp

@@ -240,10 +240,6 @@ namespace Sass {
     }
 
     else if (lex < kwd_extend >(true)) {
-      if (block->is_root()) {
-        error("Extend directives may only be used within rules.", pstate);
-      }
-
       Lookahead lookahead = lookahead_for_include(position);
       if (!lookahead.found) css_error("Invalid CSS", " after ", ": expected selector, was ");
       Selector_Obj target;