Move Delete Custom Field button to form footer

Using the HTML5 button's `formaction` property we can get rid of the
separate form that caused the button to be displayed below the Edit box,
and put the Delete button in the main form's footer

Fixes #27274

manage_custom_field_edit_page.php

@@ -372,29 +372,19 @@ class="ace" value="1"
 </div>
 </div>
 <div class="widget-toolbox padding-8 clearfix">
-	<input type="submit" class="btn btn-primary btn-white btn-round"
-		   value="<?php echo lang_get( 'update_custom_field_button' ) ?>"
-	/>
+	<button class="btn btn-primary btn-white btn-round">
+		<?php echo lang_get( 'update_custom_field_button' ) ?>
+	</button>
+	<button class="btn btn-primary btn-white btn-round"
+			formaction="manage_custom_field_delete.php">
+		<?php echo lang_get( 'delete_custom_field_button' ) ?>
+	</button>
 </div>
 </div>
 </form>
 </div>
 </div>
 
-<div class="col-md-12 col-xs-12">
-	<div class="space-10"></div>
-	<form method="post" action="manage_custom_field_delete.php" class="pull-right">
-		<fieldset>
-			<?php echo form_security_field( 'manage_custom_field_delete' ); ?>
-			<input type="hidden" name="field_id" value="<?php echo $f_field_id ?>" />
-			<input type="hidden" name="return" value="<?php echo string_attribute( $f_return ) ?>" />
-			<input type="submit" class="btn btn-primary btn-sm btn-white btn-round"
-				   value="<?php echo lang_get( 'delete_custom_field_button' ) ?>"
-			/>
-		</fieldset>
-	</form>
-</div>
-
 <?php /** @todo There is access checking in the ADD action page and at the top of this file.
 		   * We may need to add extra checks to exclude projects from the list that the user
 		   * can't link/unlink fields from/to. */