ARM: spear: remove obsolete config MACH_SPEAR600

Since commit ddb902cc ("ARM: centralize common multi-platform kconfig
options"), the config MACH_SPEAR600 has no effect. Remove this config.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20230320130159.32316-1-lukas.bulwahn@gmail.comSigned-off-by: Arnd Bergmann <arnd@arndb.de>