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>
diff --git a/configs/thunderx_88xx_defconfig b/configs/thunderx_88xx_defconfig
index 4a8655f..28797f9 100644
--- a/configs/thunderx_88xx_defconfig
+++ b/configs/thunderx_88xx_defconfig
@@ -2,7 +2,6 @@
 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> "