Revert "[TESTS] oauth2: make it possible to use an alternate http.Client"

This reverts commit 172fdd0d32.

This code was introduced for a test case that is no longer in use. It
should guard against the provider being null but that's not worth the
effort for deadcode. Just remove it.

Refs: https://codeberg.org/forgejo/forgejo/issues/1984

services/auth/source/oauth2/providers_openid.go

@@ -43,7 +43,6 @@ func (o *OpenIDProvider) CreateGothProvider(providerName, callbackURL string, so
 	if err != nil {
 		log.Warn("Failed to create OpenID Connect Provider with name '%s' with url '%s': %v", providerName, source.OpenIDConnectAutoDiscoveryURL, err)
 	}
-	provider.HTTPClient = HTTPClient
 	return provider, err
 }