board: ti: j721e: Drop SERDES PHY init from board file

Since commit 69b19ca6 ("arm: dts: k3-j721e: Sync with v6.6-rc1"),
the following error message is seen at u-boot
	"Sierra init failed:-19"

Probing and initializing the SERDES PHY from
board file is not a clean solution so drop it.

Proper use case should be via PHY_UCLASS APIs.

Signed-off-by: Roger Quadros <rogerq@kernel.org>