Remove string based named param syntax

I'm not sure this is actually useful/needed, drop it for now.
If we do support this, we nede to be careful about things like "0"
keys in additional named params.

Author: nikic

Zend/zend_language_parser.y

@@ -717,8 +717,6 @@ argument:
 		expr				{ $$ = $1; }
 	|	T_STRING T_DOUBLE_ARROW expr
 			{ $$ = zend_ast_create(ZEND_AST_NAMED_ARG, $1, $3); }
-	|	T_CONSTANT_ENCAPSED_STRING T_DOUBLE_ARROW expr
-			{ $$ = zend_ast_create(ZEND_AST_NAMED_ARG, $1, $3); }
 	|	T_ELLIPSIS expr	{ $$ = zend_ast_create(ZEND_AST_UNPACK, $2); }
 ;