[fix] preferences: opening preferences page is very slow (multiple seconds on bad hardware) (#6090)

I've been profiling the `/preferences` endpoint using werkzeug's
`ProfilerMiddleware` (i.e. just do `app.wsgi_app = ProfilerMiddleware(app.wsgi_app)`)
and look at the outputs in the terminal when doing `make run`.

It turns out that 95%+ of the time spent were inside babel's
Locale parsing (> 700ms on my machine). That's because, when opening the settings,
we loaded the full engine traits of each engine and checked if it matches
the user-defined search language. As we have 250+ engines, and babel is
very slow when parsing Locale's, this took a very long time.

By removing this feature that shows whether the selected search language
is supported by the engine, the load time went down from 800ms to 50ms
on my machine (which is still very slow, but well, that's future work on
optimizing).

searx/webapp.py

@@ -918,9 +918,6 @@ def preferences():
             'rate80': rate80,
             'rate95': rate95,
             'warn_timeout': e.timeout > settings['outgoing']['request_timeout'],
-            'supports_selected_language': e.traits.is_locale_supported(
-                str(sxng_request.preferences.get_value('language') or 'all')
-            ),
             'result_count': result_count,
         }
     # end of stats
@@ -956,15 +953,9 @@ def preferences():
     # supports
     supports = {}
     for _, e in filtered_engines.items():
-        supports_selected_language = e.traits.is_locale_supported(
-            str(sxng_request.preferences.get_value('language') or 'all')
-        )
-        safesearch = e.safesearch
-        time_range_support = e.time_range_support
         supports[e.name] = {
-            'supports_selected_language': supports_selected_language,
-            'safesearch': safesearch,
-            'time_range_support': time_range_support,
+            'safesearch': e.safesearch,
+            'time_range_support': e.time_range_support,
         }
 
     return render(