configs: Remove am43xx_evm_ethboot_defconfig

am43xx_evm_ethboot_defconfig is not being actively used and has not been
moved to DM or DT. Also, ethboot cannot be tested on AM43xx EVM as such
due EVM limitations. Therefore delete it.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>