[MM-53086] Remove SendWelcomePost A/B test and feature (#23733)

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

server/public/model/post.go

@@ -45,7 +45,6 @@ const (
 	PostTypeChannelRestored        = "system_channel_restored"
 	PostTypeEphemeral              = "system_ephemeral"
 	PostTypeChangeChannelPrivacy   = "system_change_chan_privacy"
-	PostTypeWelcomePost            = "system_welcome_post"
 	PostTypeAddBotTeamsChannels    = "add_bot_teams_channels"
 	PostTypeSystemWarnMetricStatus = "warn_metric_status"
 	PostTypeMe                     = "me"
@@ -434,7 +433,6 @@ func (o *Post) IsValid(maxPostSize int) *AppError {
 		PostTypeChangeChannelPrivacy,
 		PostTypeAddBotTeamsChannels,
 		PostTypeSystemWarnMetricStatus,
-		PostTypeWelcomePost,
 		PostTypeReminder,
 		PostTypeMe:
 	default: