Remove or fix abandoned theme implementations.

Problem/Motivation

Currently, there are two theme implementations defined in the graphql_theme() hook. They are not connected to any theme templates, which has caused fatal errors on some pages—e.g., /graphql-explorer and /graphql_voyager. I suspect there may be a correlation between core/contrib page theme suggestions and these two themes. These URLs likely attempt to render the page using the themes defined by the graphql_theme() hook, and since no templates are connected, a fatal error occurs.

The website encountered an unexpected error. Try again later.

Twig\Error\LoaderError: Template "modules/contrib/graphql/templates/page--graphql-explorer.html.twig" is not defined. in Twig\Loader\ChainLoader->getCacheKey() (line 55 of core/themes/olivero/templates/layout/html.html.twig).
Twig\Environment->getTemplateClass('modules/contrib/graphql/templates/page--graphql-explorer.html.twig', NULL) (Line: 204)
Drupal\Core\Template\TwigEnvironment->getTemplateClass('modules/contrib/graphql/templates/page--graphql-explorer.html.twig') (Line: 371)
Twig\Environment->load('modules/contrib/graphql/templates/page--graphql-explorer.html.twig') (Line: 33)
twig_render_template('modules/contrib/graphql/templates/page--graphql-explorer.html.twig', Array) (Line: 348)
Drupal\Core\Theme\ThemeManager->render('page', Array) (Line: 446)

Proposed resolution

Since there doesn’t appear to be any usage of these theme implementations, I believe they should be removed.