MM-22051: Remove FromJson part 2 (#17981)

```release-note
Removed the following functions:
ChannelsWithCountFromJson
ChannelFromJson
ChannelPatchFromJson
ChannelModerationsFromJson
ChannelModerationsPatchFromJson
ChannelMemberCountsByGroupFromJson
ChannelCountsFromJson
ChannelDataFromJson
ChannelListFromJson
ChannelSliceFromJson
ChannelListWithTeamDataFromJson
ChannelMembersFromJson
ChannelUnreadFromJson
ChannelUnreadAtFromJson
ChannelMemberFromJson
ChannelSearchFromJson
SidebarCategoryFromJson
SidebarCategoriesFromJson
OrderedSidebarCategoriesFromJson
```

https://mattermost.atlassian.net/browse/MM-22051

api4/data_retention.go

@@ -302,8 +302,9 @@ func getChannelsForPolicy(c *Context, w http.ResponseWriter, r *http.Request) {
 
 func searchChannelsInPolicy(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.RequirePolicyId()
-	props := model.ChannelSearchFromJson(r.Body)
-	if props == nil {
+	var props *model.ChannelSearch
+	err := json.NewDecoder(r.Body).Decode(&props)
+	if err != nil {
 		c.SetInvalidParam("channel_search")
 		return
 	}