configs: omap4: remove unused boot target devices

configs: omap4: remove unused boot target devices

This include file isn't used since the omap4 support has been dropped.
Since this support is about to be reintroduced, this file is going to be
used again. The upcoming support is minimal and doesn't include network,
therefore leaving PXE and DHCP in the BOOT_TARGET_DEVICES list would lead
to build errors.

Remove PXE and DHCP from the list of BOOT_TARGET_DEVICES.
Remove the LEGACY_MMC macros and the findfdt script that looks for
no-longer supported boards.
Remove the empty #ifdef XPL_BUILD

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>