oidc: Remove "aud" claim validation of logout tokens (#6156)

The "aud" claim of the logout token is supposed to contain the client-id
of the client for which the token was issued. Our current implementation of
validating that claim is somewhat broken. We only allow to configure a single
value for the allowed client id. But we have different client-ids
accessing oCIS.

This completely removes the current validation of the `aud` claim until
we come up with a working solution. As we currently require a session id
to be present in the logout token the risk not validating the `aud`
claim is pretty low.

Related: #6149

services/proxy/pkg/config/config.go

@@ -108,8 +108,6 @@ type OIDC struct {
 	UserinfoCache           *Cache `yaml:"user_info_cache"`
 	JWKS                    JWKS   `yaml:"jwks"`
 	RewriteWellKnown        bool   `yaml:"rewrite_well_known" env:"PROXY_OIDC_REWRITE_WELLKNOWN" desc:"Enables rewriting the /.well-known/openid-configuration to the configured OIDC issuer. Needed by the Desktop Client, Android Client and iOS Client to discover the OIDC provider."`
-	ClientID                string `yaml:"client_id" env:"OCIS_OIDC_CLIENT_ID;PROXY_OIDC_CLIENT_ID" desc:"OIDC client ID, which ownCloud Web uses. This client needs to be set up in your external IDP (has no effect when using the builtin IDP)."`
-	SkipClientIDCheck       bool   `yaml:"skip_client_id_check" env:"PROXY_OIDC_SKIP_CLIENT_ID_CHECK" desc:"If true will skip checking the configured client ID is present in audience claims. See following chapter for more details: https://openid.net/specs/openid-connect-core-1_0.html#IDToken"`
 }
 
 type JWKS struct {