Remove deprecated slack plugin credentials (#48702) (#48818)

Follow up to #48596
to remove the deprecated field now that
gravitational/teleport.e#5373 has removed
the last usages.

lib/config/configuration_test.go

@@ -3662,10 +3662,6 @@ func TestAuthHostedPlugins(t *testing.T) {
 			applyErr: require.NoError,
 			assert: func(t *testing.T, p servicecfg.HostedPluginsConfig) {
 				require.True(t, p.Enabled)
-				require.NotNil(t, p.OAuthProviders.Slack)
-				require.Equal(t, "foo", p.OAuthProviders.Slack.ID)
-				require.Equal(t, "bar", p.OAuthProviders.Slack.Secret)
-
 				require.NotNil(t, p.OAuthProviders.SlackCredentials)
 				require.Equal(t, "foo", p.OAuthProviders.SlackCredentials.ClientID)
 				require.Equal(t, "bar", p.OAuthProviders.SlackCredentials.ClientSecret)

lib/config/fileconf.go

@@ -33,7 +33,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/coreos/go-oidc/oauth2"
 	"github.com/gravitational/trace"
 	log "github.com/sirupsen/logrus"
 	"golang.org/x/crypto/acme"
@@ -1334,10 +1333,6 @@ func (p *PluginOAuthProviders) Parse() (servicecfg.PluginOAuthProviders, error)
 		if err != nil {
 			return out, trace.Wrap(err)
 		}
-		out.Slack = &oauth2.ClientCredentials{
-			ID:     slack.ClientID,
-			Secret: slack.ClientSecret,
-		}
 		out.SlackCredentials = slack
 	}
 	return out, nil

lib/service/servicecfg/auth.go

@@ -19,7 +19,6 @@
 package servicecfg
 
 import (
-	"github.com/coreos/go-oidc/oauth2"
 	"github.com/dustin/go-humanize"
 	"github.com/gravitational/trace"
 	"github.com/jonboulle/clockwork"
@@ -180,10 +179,6 @@ type HostedPluginsConfig struct {
 // PluginOAuthProviders holds application credentials for each
 // 3rd party API provider
 type PluginOAuthProviders struct {
-	// TODO(tross) delete once teleport.e has been converted.
-	// Deprecated: use SlackCredentials instead.
-	Slack *oauth2.ClientCredentials
-
 	SlackCredentials *OAuthClientCredentials
 }