AT91: Drop AT91_SPIMUX command from cmd_all

$ git grep AT91_SPIMUX
include/config_cmd_all.h:#define CONFIG_CMD_AT91_SPIMUX /* AT91 MMC/SPI Mux Support     */

It isn't used anywhere it seems.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Anatolij Gustschin <agust@denx.de>

include/config_cmd_all.h

@@ -15,7 +15,6 @@
 
 #define CONFIG_CMD_AMBAPP	/* AMBA Plug & Play Bus print utility */
 #define CONFIG_CMD_ASKENV	/* ask for env variable		*/
-#define CONFIG_CMD_AT91_SPIMUX	/* AT91 MMC/SPI Mux Support     */
 #define CONFIG_CMD_BDI		/* bdinfo			*/
 #define CONFIG_CMD_BEDBUG	/* Include BedBug Debugger	*/
 #define CONFIG_CMD_BMP		/* BMP support			*/