Form: Simplify player filtering

The game.users provides a .players member, listing only non-GM
users.

Use this instead of filtering the list manually.

scripts/send_dialog_form.mjs

@@ -43,7 +43,7 @@ export function validate_form(form_response) {
  */
 export function prepare_send_message_html(existing_values = null, chain = false) {
     let players_options = []
-    for (let user of game.users.filter((user) => !user.isGM)) {
+    for (let user of game.users.players) {
         players_options.push({
             label: user.character ? user.character.name : user.name,
             value: user.id,