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

model/channel_data.go

@@ -5,7 +5,6 @@ package model
 
 import (
 	"encoding/json"
-	"io"
 )
 
 type ChannelData struct {
@@ -26,9 +25,3 @@ func (o *ChannelData) ToJson() string {
 	b, _ := json.Marshal(o)
 	return string(b)
 }
-
-func ChannelDataFromJson(data io.Reader) *ChannelData {
-	var o *ChannelData
-	json.NewDecoder(data).Decode(&o)
-	return o
-}