fix(test_reboot): do not assert thread count

An upstream patch was backported to ubuntu 24.04 6.8.0-58 kernel that
makes the nx hugepages recover thread a child of the firecracker
process, thus increasing process count to 7.
As we're not really interested in knowing how many threads we have in
this test, let's remove the assertion altogether.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

tests/integration_tests/functional/test_shut_down.py

@@ -27,15 +27,6 @@ def test_reboot(uvm_plain_any):
     vm.add_net_iface()
     vm.start()
 
-    # Get Firecracker PID so we can count the number of threads.
-    firecracker_pid = vm.firecracker_pid
-
-    # Get number of threads in Firecracker
-    cmd = "ps -o nlwp {} | tail -1 | awk '{{print $1}}'".format(firecracker_pid)
-    _, stdout, _ = utils.check_output(cmd)
-    nr_of_threads = stdout.rstrip()
-    assert int(nr_of_threads) == 6
-
     # Consume existing metrics
     lines = vm.get_all_metrics()
     assert len(lines) == 1