Revert "dsa: mv88e6xxx: make serdes SGMII/Fiber tx amplitude configurable"

Revert "dsa: mv88e6xxx: make serdes SGMII/Fiber tx amplitude configurable"

This reverts commit 926eae604403acfa27ba5b072af458e87e634a50, which
never could have produced the intended effect:
https://lore.kernel.org/netdev/AM0PR06MB10396BBF8B568D77556FC46F8F7DEA@AM0PR06MB10396.eurprd06.prod.outlook.com/

The reason why it is broken beyond repair in this form is that the
mv88e6xxx driver outsources its "tx-p2p-microvolt" property to the OF
node of an external Ethernet PHY. This:
(a) does not work if there is no external PHY (chip-to-chip connection,
    or SFP module)
(b) pollutes the OF property namespace / bindings of said external PHY
    ("tx-p2p-microvolt" could have meaning for the Ethernet PHY's SerDes
    interface as well)

We can revisit the idea of making SerDes amplitude configurable once we
have proper bindings for the mv88e6xxx SerDes. Until then, remove the
code that leaves us with unnecessary baggage.

Fixes: 926eae604403 ("dsa: mv88e6xxx: make serdes SGMII/Fiber tx amplitude configurable")
Cc: Holger Brunck <holger.brunck@hitachienergy.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260104093952.486606-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>