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>