tree-wide: drop several remaining license headers

And downgrade the license of utf8.c to LGPL-2.0-or-later, to follow the
original license.

src/basic/utf8.c

@@ -1,26 +1,10 @@
-/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/* SPDX-License-Identifier: LGPL-2.0-or-later */
 
-/* Parts of this file are based on the GLIB utf8 validation functions. The
- * original license text follows. */
-
-/* gutf8.c - Operations on UTF-8 strings.
+/* Parts of this file are based on the GLIB utf8 validation functions. The original copyright follows.
  *
+ * gutf8.c - Operations on UTF-8 strings.
  * Copyright (C) 1999 Tom Tromey
  * Copyright (C) 2000 Red Hat, Inc.
- *
- * This library is free software; you can redistribute it and/or
- * modify it under the terms of the GNU Library General Public
- * License as published by the Free Software Foundation; either
- * version 2 of the License, or (at your option) any later version.
- *
- * This library is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
- * Library General Public License for more details.
- *
- * You should have received a copy of the GNU Library General Public
- * License along with this library; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
 #include <errno.h>