Remove performance link from features nav

The features nav is supposed to navigate your between pages in the app.
It’s very unexpected to have it open an external link.

Performance isn’t strictly a part of Support, but it’s worked having it
there for long enough that it’s probably not a bother.

app/main/views/sub_navigation_dictionaries.py

@@ -12,11 +12,6 @@ def features_nav():
             "name": "Security",
             "link": "main.security",
         },
-        {
-            "name": "Performance",
-            "link": "https://www.gov.uk/performance/govuk-notify",
-            "external_link": True,
-        },
         {
             "name": "Terms of use",
             "link": "main.terms",