[cfg] fix bogus constraint on ':=' operator

It is actually implemented by BAVP also (not only AVPs), but the script does not allow its usage :(.
Removing the check is safe as all the other variables will treat the ':=' as '=' - they do not test the operator during "set", assuming '=' all the time.

(cherry picked from commit 5bab986)

cfg.y

@@ -1997,13 +1997,6 @@ assign_cmd: script_var assignop assignexp {
 	|  script_var COLONEQ NULLV {
 			if(!pv_is_w($1))
 				yyerror("invalid left operand in assignment");
-			/* not all can get NULL with := */
-			switch($1->type) {
-				case PVT_AVP:
-				break;
-				default:
-					yyerror("invalid left operand in NULL assignment");
-			}
 			if($1->trans!=0)
 				yyerror("transformations not accepted in left side "
 					"of assignment");