platforms.nix: Clean up more 'uboot' legacy

For a while now, the only thing the 'uboot' attribute does is to tell
whether to add ubootTools to kernel/initrd builds. That can be
determined with platform.kernelTarget == "uImage" just as well.

pkgs/os-specific/linux/kernel/generic.nix

@@ -29,7 +29,6 @@
 , ...
 }:
 
-assert stdenv.platform.name == "sheevaplug" -> stdenv.platform.uboot != null;
 assert stdenv.isLinux;
 
 let