board: stm32mp1: remove board.c

Remove the file board/st/stm32mp1/board.c which is not more
compiled since commit 156732cc8939 ("board: stm32mp1: move the
function board_debug_uart_init in spl.c")

Fixes: 4fb46816 ("board: stm32mp1: move the function board_debug_uart_init in spl.c")
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>