util: get rid of json_array_append

util: get rid of json_array_append

This function is causing a memleak because it is necessary to clean
up after usage.

This fixes at least:

37 (32 direct, 5 indirect) bytes in 1 blocks are definitely lost in loss record 104 of 394
   at 0x4C29C0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x66D0C7B: ??? (in /usr/lib/x86_64-linux-gnu/libjansson.so.4.7.0)
   by 0x943584: LiveDeviceIfaceList (util-device.c:264)
   by 0x910889: UnixCommandExecute (unix-manager.c:486)
   by 0x910D7E: UnixCommandRun (unix-manager.c:545)
   by 0x911193: UnixMain (unix-manager.c:593)
   by 0x913CC7: UnixManager (unix-manager.c:961)
   by 0x907753: TmThreadsManagement (tm-threads.c:600)
   by 0x68DE283: start_thread (pthread_create.c:333)
   by 0x80A6A4C: clone (in /lib/x86_64-linux-gnu/libc-2.21.so)