net: stmmac: remove speed_mode_2500() method

Remove the speed_mode_2500() platform method which is no longer used
or necessary, being superseded by the more flexible get_interfaces()
method.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uASM3-0021R3-2B@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>