stm32: drop empty usart.c files

On some targets these weren't even being referenced, on others they
were, in all cases they were empty and contributed no value.

They _actually_ served to declare groupings for doxygen, but we can do
that in a different manner, without having to have dummy files around.

lib/stm32/f1/usart.c

@@ -1,31 +0,0 @@
-/** @defgroup usart_file USART
-
-@ingroup STM32F1xx
-
-@brief <b>libopencm3 STM32F1xx USART</b>
-
-@version 1.0.0
-
-@date 30 August 2012
-
-LGPL License Terms @ref lgpl_license
-*/
-
-/*
- * This file is part of the libopencm3 project.
- *
- * This library is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Lesser General Public License as published by
- * the Free Software Foundation, either version 3 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 Lesser General Public License for more details.
- *
- * You should have received a copy of the GNU Lesser General Public License
- * along with this library.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-#include <libopencm3/stm32/usart.h>