Use matterbridge vendored slack

vendor/github.com/nlopes/slack/comment.go

@@ -1,10 +0,0 @@
-package slack
-
-// Comment contains all the information relative to a comment
-type Comment struct {
-	ID        string   `json:"id,omitempty"`
-	Created   JSONTime `json:"created,omitempty"`
-	Timestamp JSONTime `json:"timestamp,omitempty"`
-	User      string   `json:"user,omitempty"`
-	Comment   string   `json:"comment,omitempty"`
-}