pico-imx7d: Remove board_phy_config()

pico-imx7d: Remove board_phy_config()

With Ethernet DM in place, there is no longer the need for having
the board_phy_config() anymore.

Remove it.

Tested on imx7d-pico-pi board:

=> setenv autoload no
=> dhcp
BOOTP broadcast 1
*** Unhandled DHCP Option in OFFER/ACK: 42
DHCP client bound to address 192.168.0.138 (138 ms)
=> tftp zImage
Using ethernet@30be0000 device
TFTP from server 192.168.0.16; our IP address is 192.168.0.138
Filename 'zImage'.
Load address: 0x80800000
Loading: #################################################################
....
	 #################################################################
	 ##########
	 4.3 MiB/s
done
Bytes transferred = 9034120 (89d988 hex)

Signed-off-by: Fabio Estevam <festevam@denx.de>