arm64: defconfig: remove obsolete CONFIG_ARCH_AGILEX and CONFIG_ARCH_N5X

These config options were removed in 4a9a1a56 ("arm64: socfpga: merge
Agilex and N5X into ARCH_INTEL_SOCFPGA"), so remove them from the
defconfig.

Fixes: 4a9a1a56 ("arm64: socfpga: merge Agilex and N5X into ARCH_INTEL_SOCFPGA")
Signed-off-by: Ross Burton <ross.burton@arm.com>