efi_loader: don't use memmove() in efi_var_mem_del()

efi_var_mem_del() is in __efi_runtime because it would be needed for a
runtime implementation of SetVariable(). memmove() is not in __efi_runtime.
So we should not use it in efi_var_mem_del().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>