Remove X-Slack-Channel from email headers

Turns out, gmail doesn't preserve this when replying to messages.

Signed-off-by: Anna Schumaker <Anna@OcarinaProject.net>

slackmail.py

@@ -19,11 +19,11 @@ for channel in slack.channels.list():
     for message in channel.read():
         text += [ str(message) ]
 
+    text += [ "", "--- Reply above this line ---", "channel=%s" % channel.id() ]
     print("\n".join(text))
     print()
 
     msg = MIMEText("\n".join(text))
-    msg["X-Slack-Channel"] = channel.id()
     msg["Subject"] = "[%s] %s" % (slack.team(), channel.name())
     msg["From"] = "SlackMail@OcarinaProject.net"
     msg["To"] = slack.user().email()