arch: arm: aarch64: remove non-applicable linker section

The non-secure callable functions' section is only applicable
to Cortex-M with TrustZone-M extension. Remove it from AARCH64
linker script. (CONFIG_ARM_FIRMWARE_HAS_SECURE_ENTRY_FUNCTIONS
is only enabled for Cortex-M so this is a no-op, but still, it
is a useful cleanup.)

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>