sched: Remove task_delete in case of CONFIG_BUILD_KERNEL

Deleting a task from another task's context will not do, so shut
this gate down for BUILD_KERNEL. In this case if a task wants another
task to terminate, it must ask the other task to politely kill itself.

Note: kthreads still need this, also, the kernel can delete a task
without asking.

include/sys/syscall_lookup.h

@@ -92,10 +92,11 @@ SYSCALL_LOOKUP(sem_wait,                   1)
 #ifndef CONFIG_BUILD_KERNEL
   SYSCALL_LOOKUP(task_create,              5)
   SYSCALL_LOOKUP(task_spawn,               6)
+  SYSCALL_LOOKUP(task_delete,              1)
 #else
   SYSCALL_LOOKUP(pgalloc,                  2)
 #endif
-SYSCALL_LOOKUP(task_delete,                1)
+
 SYSCALL_LOOKUP(task_restart,               1)
 SYSCALL_LOOKUP(task_setcancelstate,        2)
 SYSCALL_LOOKUP(up_assert,                  2)