[mod] drop SearXNG's checker (#5767)

To date, there is no analysis for the checker that could be evaluated in any
meaningful way.

- https://github.com/searxng/searxng/issues/3407
- https://github.com/searxng/searxng/pull/3312

The checker would need to be completely redesigned, but even then, its
usefulness and the maintenance required for it would be disproportionate.

TBH: In its current form, it is useless and only consumes resources and
causes the engines to be blocked, because these tests (query terms) come
from *hundreds* of instances and could be interpreted as bot attacks.

Related issues: [search.checker](https://github.com/searxng/searxng/issues?q=label%3A%22search.checker%22)

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings_defaults.py

@@ -267,10 +267,6 @@ SCHEMA: dict[str, t.Any] = {
         'networks': {},
     },
     'plugins': SettingsValue(dict, {}),
-    'checker': {
-        'off_when_debug': SettingsValue(bool, True, None),
-        'scheduling': SettingsValue((None, dict), None, None),
-    },
     'categories_as_tabs': SettingsValue(dict, CATEGORIES_AS_TABS),
     'engines': SettingsValue(list, []),
     'doi_resolvers': {},