arm64: thunderx_88xx_defconfig: remove unneeded CONFIG_SYS_EXTRA_OPTIONS

ARM64 is correctly select'ed in arch/arm/Kconfig, so this line in
the defconfig is unneeded.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

configs/thunderx_88xx_defconfig

@@ -2,7 +2,6 @@ CONFIG_ARM=y
 CONFIG_TARGET_THUNDERX_88XX=y
 CONFIG_DM_SERIAL=y
 CONFIG_DEFAULT_DEVICE_TREE="thunderx-88xx"
-CONFIG_SYS_EXTRA_OPTIONS="ARM64"
 CONFIG_BOOTDELAY=5
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="ThunderX_88XX> "