treewide: mem: Remove alternate memtest setup from Mox and x530

The commit e89f8aae ("treewide: Migrate CONFIG_SYS_ALT_MEMTEST to Kconfig")
setup correct dependency on MEMTEST that's why there is no reason to enable
enable alternate memtest without mtest command.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>