[MM-51274] Remove deprecated PermissionUseSlashCommands (#22819)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/app_test.go

@@ -119,7 +119,6 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 			model.PermissionGetPublicLink.Id,
 			model.PermissionCreatePost.Id,
 			model.PermissionUseChannelMentions.Id,
-			model.PermissionUseSlashCommands.Id,
 			model.PermissionManagePublicChannelProperties.Id,
 			model.PermissionDeletePublicChannel.Id,
 			model.PermissionManagePrivateChannelProperties.Id,