sunxi: remove bogus mmc_pinmux_setup() prototype

Since all callers of mmc_pinmux_setup() are located after the definition
of that function, there is no need for a forward declaration (anymore?).

Remove the prototype along with its #ifdef guards.

Reviewed-by: Samuel Holland <samuel@sholland.org>
Tested-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/board/sunxi/board.c b/board/sunxi/board.c
index 4d2491b..4fe749f 100644
--- a/board/sunxi/board.c
+++ b/board/sunxi/board.c
@@ -184,10 +184,6 @@
 	return ENVL_UNKNOWN;
 }
 
-#ifdef CONFIG_DM_MMC
-static void mmc_pinmux_setup(int sdc);
-#endif
-
 /* add board specific code here */
 int board_init(void)
 {