contest: hw: fix sol_listener linter warning

GH linter throws a warning:

  contest/hw/machine_control.py:417:5: F824 `global sol_listener` is unused: name is never assigned in scope
    global db_pool, health_checker, res_mgr, sol_listener  # pylint: disable=global-statement
    ^
     F824 `global sol_listener` is unused: name is never assigned in scope

which neither pylint not ruff check produces locally.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

contest/hw/machine_control.py

@@ -414,7 +414,7 @@ def reservation_close():
 
 def main():
     """Initialize services and run Flask app."""
-    global db_pool, health_checker, res_mgr, sol_listener  # pylint: disable=global-statement
+    global db_pool, health_checker, res_mgr  # pylint: disable=global-statement
 
     config = configparser.ConfigParser()
     cfg_paths = ['hw.config', 'machine_control.config']