performance: Cleanup code meant for cockpit version < 235

Bullseye and higher has version 239 or higher.

Tests:

- From performance app, launch the web interface in a testing container. Web
interface shows up properly.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/performance/manifest.py

@@ -3,36 +3,12 @@
 FreedomBox app for System Monitoring (cockpit-pcp) in ‘System’.
 """
 
-import subprocess
-from functools import lru_cache
-
-from django.utils.functional import lazy
 from django.utils.translation import gettext_lazy as _
 
-from plinth.utils import Version
-
-
-@lru_cache()
-def _get_url():
-    """Return the web client URL based on Cockpit version."""
-    process = subprocess.run(
-        ['dpkg-query', '--showformat=${Version}', '--show', 'cockpit'],
-        stdout=subprocess.PIPE)
-    cockpit_version = process.stdout.decode()
-    if Version(cockpit_version) >= Version('235'):
-        url = '/_cockpit/metrics'
-    else:
-        url = '/_cockpit/system/graphs'
-
-    return url
-
-
-get_url = lazy(_get_url, str)
-
 clients = [{
     'name': _('Cockpit'),
     'platforms': [{
         'type': 'web',
-        'url': get_url()
+        'url': '/_cockpit/metrics'
     }]
 }]