[Kanegasi]

ActionButton1.Border = nil

Using hide won't work, since the code responsible for the green border will show it again.

\FrameXML\ActionButton.lua

-- Add a green border if button is an equipped item
local border = self.Border;
if border then
    if ( IsEquippedAction(action) ) then
        border:SetVertexColor(0, 1.0, 0, 0.35);
        border:Show();
    else
        border:Hide();
    end
end