efi/libstub: remove unnecessary cmd_line_len from efi_convert_cmdline()

efi_convert_cmdline() always sets cmdline_size to at least 1 on success,
so the "cmdline_size > 0" does nothing and can be removed (the intent
was to avoid parsing an empty string, but there is nothing wrong with
parsing an empty string, it is only making boot negligibly slower).

Then the cmd_line_len argument to efi_convert_cmdline can be removed
because there is nothing left using it.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>