plat/kvm/x86: Remove mrd allocation of cmdline

plat/kvm/x86: Remove mrd allocation of cmdline

Remove cmdline mrd allocation from the efi, lxboot, multiboot boot
protocols as this requirement has been deprected by libukboot early
init.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Oleksii Moisieiev <oleksii_moisieiev@epam.com>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Stan <andreistan2003@gmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
GitHub-Closes: #1472