sample: Remove unused field

Signed-off-by: Ikey Doherty <ikey@aerynos.com>

Author: ikeycode

sample.json

@@ -55,7 +55,6 @@
         "strategy": "immediate",
         "rollback_support": true
       },
-      "composite": true,
       "boot": {
         "bootloader": "systemd-boot",
         "firmware": {