emulator_arm64: Revert TARGET_NO_KERNEL to emulator default

With TARGET_NO_KERNEL set to false, sdk_phone_arm64 build depends on
$PRODUCT_OUT/kernel, which no longer exists as of aosp/2548555 ("Cleanup
in emulator_arm64/device.mk").

$ lunch sdk_phone_arm64-eng && m
[...]
FAILED: ninja: 'out/target/product/emulator_arm64/kernel', needed by
  'out/target/product/emulator_arm64/boot.img', missing and no known
  rule to make it

Goldfish makefiles copy prebuilt kernels to $OUT/kernel-ranchu that
the emulator then uses, so no need to generate the boot image.

Test: lunch sdk_phone_arm64-eng && m
Change-Id: I3598ac04b352fc07679315ced4f7b7a536ccb5a5