conf: machine: imx8m*: clean-up obsolete ATF load address
Upstream U-Boot commit
d9a6f0eed66a ("tree: imx: remove old fit
generator script") dropped the FIT generator script, which was using
environment variable setting the ATF load address into the FIT ITS file.
This has been replaced by binman node description, where ATF address is
defined, hence the enviroment variable is not required anymore.
Clean-up the layer's class and machine description to remove the
variable, which is not used anymore.
Signed-off-by: Andrey Zhizhikin <andrey.z@gmail.com>
