Remove guard around opening i18n.json

This operation should never fail because the i18n.json file is
shipped with the package and always present.

Possibly the rationale of needing it for tests no longer applies
since the path is now absolute and not relative to the current
working directory.

See <#7443 (comment)>.

securedrop/i18n.py

@@ -142,13 +142,9 @@ def validate_locale_configuration(config: SecureDropConfig, babel: Babel) -> Set
     available.add(Locale.parse(FALLBACK_LOCALE))
 
     # These locales are supported in the current version of securedrop-app-code.
-    try:
-        with open(I18N_CONF) as i18n_conf_file:
-            i18n_conf = json.load(i18n_conf_file)
-        supported = parse_locale_set(i18n_conf["supported_locales"].keys())
-    # I18N_CONF may not be available under test.
-    except FileNotFoundError:
-        supported = available
+    with open(I18N_CONF) as i18n_conf_file:
+        i18n_conf = json.load(i18n_conf_file)
+    supported = parse_locale_set(i18n_conf["supported_locales"].keys())
 
     # These locales were configured via "securedrop-admin sdconfig", meaning
     # they were present on the Admin Workstation at "securedrop-admin" runtime.