verdin-imx8mm: Remove stale fdt_addr env variable

fdt_addr variable is the location in flash of the device tree blob [1], it
does not exist for verdin-imx8mm.

Because of this the bootefi command fails unless the optional
`[fdt address]` parameter is passed on the command line,
bootefi.c:efi_install_fdt() assumes that `fdt_addr` is valid when
present.

Fix this removing fdt_addr from the U-Boot environment.

[1] doc/usage/environment.rst

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>