平成30/ 2018-10-22 00:47
javascript/ replace

<a onclick="
var str = document.allfield_fm.allfield.value;
document.allfield_fm.allfield.value=str.replace('フリーワード検索', '');
document.allfield_fm.submit();
return false; " href="javascript:void(0);">

( Thanks. )
JavaScript 文字列の置き換え - replace/replaceAll
http://www.syboos.jp/webjs/doc/string-replace-and-replaceall.html