Revert "MM-16809: Added EnsureChannel Helper (#11852)" (#12314)

This reverts commit 5aa24aedc8.

plugin/client.go

@@ -10,7 +10,6 @@ import (
 const (
 	INTERNAL_KEY_PREFIX = "mmi_"
 	BOT_USER_KEY        = INTERNAL_KEY_PREFIX + "botid"
-	CHANNEL_KEY         = INTERNAL_KEY_PREFIX + "channelid"
 )
 
 // Starts the serving of a Mattermost plugin over net/rpc. gRPC is not yet supported.