target/arm: Remove duplicate 'host' entry in '-cpu ?' output

Since 86f0a186 the TYPE_ARM_HOST_CPU is only compiled when CONFIG_KVM
is enabled.

Remove the now redundant special-case introduced in a96c0514, to avoid:

  $ qemu-system-aarch64 -machine virt -cpu \? | fgrep host
  host
  host (only available in KVM mode)

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180727132311.2777-1-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>