Unfortunately, you have no control over the appearance of the floating panel in a message view window. You can replace the default | Reply | Reply All | Forward | button array in the toolbar with just | Reply | Forward | tools — but this has no bearing on the default floating button panel at the top of the message view window. Those are hardwired into the Mail program and cannot be changed.
Click and drag the default Reply, Reply All, and Forward button cluster from the toolbar out off of the Mail program. It will vanish. From the View menu : Customize Toolbar… locate the separate Reply and Forward icons, and drag, position, and drop them in the same toolbar location.
You can retrain yourself to use the main Mail toolbar icons after you have replaced as above, rather than the floating panel. There is no toolbar item for message attachments, and you will still need to rely on the floating message panel right-most icon for that disposition.
Clicking the Reply All button will send your email response to all email addresses (including visible/hidden Mail groups) that appear in the original Mail header fields. Think of it as the "I want to deliberately embarass myself" button.
