nsh_vars: Remove rogue sched_unlock()

There is no corresponding sched_lock() for this and therefore this causes
random crashes.

Found when running a long init script which utilizes shell variables in
SMP mode.

nshlib/nsh_vars.c

@@ -346,7 +346,6 @@ int nsh_unsetvar(FAR struct nsh_vtbl_s *vtbl, FAR const char *name)
         }
     }
 
-  sched_unlock();
   return ret;
 }
 #endif