delete unused template, re #519

templates/sidebar/options-widget-form.php

-<form class="default" action="<?= URLHelper::getLink($form_action_url) ?>"
-      method="<?= htmlReady($form_method) ?>">
-    <? if ($form_method == 'post'): ?>
-        <?= CSRFProtection::tokenTag() ?>
-    <? endif ?>
-    <? foreach ($option_elements as $element): ?>
-        <? if ($element['type'] == 'checkbox'): ?>
-            <input type="checkbox" name="<?= htmlReady($element['name']) ?>"
-                   value="<?= htmlReady($element['value']) ?>"
-                   <?= $element['checked'] ? 'checked="checked"' : '' ?>>
-            <label for="<?= htmlReady($element['name']) ?>">
-                <?= htmlReady($element['label']) ?>
-            </label>
-        <? elseif ($element['type'] == 'radio'): ?>
-                <input type="checkbox" name="<?= htmlReady($element['name']) ?>"
-                       value="<?= htmlReady($element['value']) ?>"
-                       <?= $element['checked'] ? 'checked="checked"' : '' ?>>
-            <label for="<?= htmlReady($element['name']) ?>">
-                    <?= htmlReady($element['label']) ?>
-            </label>
-        <? elseif ($element['type'] == 'select'): ?>
-            <label>
-                <?= htmlReady($element['label']) ?>
-                <select name="<?= htmlReady($element['name']) ?>" aria-label="<?= htmlReady($title) ?>">
-                    <? foreach ($element['options'] as $key => $name): ?>
-                        <option value="<?= htmlReady($key) ?>">
-                            <?= htmlReady($name) ?>
-                        </option>
-                    <? endforeach ?>
-                </select>
-            </label>
-        <? endif ?>
-    <? endforeach ?>
-    <? if (!$submit_form_directly): ?>
-        <?= \Studip\Button::create(_('Setzen'), 'set') ?>
-    <? endif ?>
-</form>