MM-22051: Remove ToJson from model (part 1) (#18005)

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

```release-note
Removed the following methods/functions:
(ad *AccessData) ToJson()
(ar *AccessResponse) ToJson()
(ar *AnalyticsRow) ToJson()
(ar AnalyticsRows) ToJson()
(o *Audit) ToJson()
(o Audits) ToJson()
(ad *AuthData) ToJson()
(ar *AuthorizeRequest) ToJson()
(o *ChannelPatch) ToJson()
(o *ChannelsWithCount) ToJson()
(o *ChannelCounts) ToJson()
(o *ChannelData) ToJson()
(o *ChannelMembers) ToJson()
(o *ChannelUnread) ToJson()
(o *ChannelUnreadAt) ToJson()
(o *ChannelStats) ToJson()
(o *ChannelView) ToJson()
(o *ChannelViewResponse) ToJson()
(o *ClusterDiscovery) ToJson()
(ci *ClusterInfo) ToJson()
(cs *ClusterStats) ToJson()
(o *Command) ToJson()
CommandListToJson(l []*Command) string
(o *CommandArgs) ToJson()
(cmr *CommandMoveRequest) ToJson()
(o *CommandResponse) ToJson()
(c *Compliance) ToJson()
(c Compliances) ToJson()
(o *Config) ToJson()
EmojiListToJson(emojiList []*Emoji)
```

cmd/mattermost/commands/cmdtestlib.go

@@ -5,6 +5,7 @@ package commands
 
 import (
 	"bytes"
+	"encoding/json"
 	"flag"
 	"fmt"
 	"io"
@@ -122,7 +123,11 @@ func (h *testHelper) SetConfig(config *model.Config) {
 
 	h.config = config
 
-	if err := ioutil.WriteFile(h.configFilePath, []byte(config.ToJson()), 0600); err != nil {
+	buf, err := json.Marshal(config)
+	if err != nil {
+		panic("failed to marshal config: " + err.Error())
+	}
+	if err := ioutil.WriteFile(h.configFilePath, buf, 0600); err != nil {
 		panic("failed to write file " + h.configFilePath + ": " + err.Error())
 	}
 }