Don't send a bot reply when calling `&cj pin`

[shenanigansd]

• Requested here [requires events role]; and tentatively approved here [requires events role]

When calling &cj pin, you get the system message of "Sir Robin pinned a message to this channel.", and then you get Sir Robin himself confirming that ":white_check_mark: Message has been pinned."

The original request was to make the bot message ephemeral, but I believe that it can removed entirely.
All we want to do is send a visual confirmation that the command was received, and that is accomplished by the system message.