ARM: mstar: remove unused config MACH_MERCURY

ARM: mstar: remove unused config MACH_MERCURY

Commit 312b62b6610c ("ARM: mstar: Add machine for MStar/Sigmastar Armv7
SoCs") adds the config MACH_INFINITY and MACH_MERCURY. The MACH_INFINITY
config is used in the gpio-msc313 driver, but the MACH_MERCURY config is
never used to configure anything in the kernel tree.

Remove the unused config MACH_MERCURY.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Romain Perier <romain.perier@gmail.com>
Link: https://lore.kernel.org/r/20230321033810.22017-1-lukas.bulwahn@gmail.com
Signed-off-by: Arnd Bergmann <arnd@arndb.de>