board: beagle: beagleplay: env: Drop usb and pxe as boot targets

We had enabled USB and network pxe boot with the hope to get it all
merged on time. However, it has not panned out. Drop usb and pxe boot
else bootflow scan -l throws in:
a) Unknown uclass 'usb' in label
b) Crashes when attempting pxe - cpsw/mdio driver apparently has missing
   error handling of what ever form. This is the one that Jan noticed in
   his log.

We can enable these on a later date once things are working.

Cc: Roger Quadros <rogerq@kernel.org>

Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Closes: https://lore.kernel.org/all/782ea2c0-eef5-478d-a122-cc6e2d066762@siemens.com/


Signed-off-by: Nishanth Menon <nm@ti.com>