refactor(shared): remove obsolete dropdown widgets

Deletes the `CountryDropdownFormField` and `LanguageDropdownFormField`
widgets as they have been fully replaced by the new, more capable
`SearchableDropdownFormField` across the application.

Updates the shared widgets barrel file to remove the exports for the
deleted files.

Author: fulleni

lib/shared/widgets/country_dropdown_form_field.dart

@@ -1,3 +1 @@
-export 'country_dropdown_form_field.dart';
-export 'language_dropdown_form_field.dart';
 export 'searchable_dropdown_form_field.dart';