x86/efi: Remove references of EFI earlyprintk from documentation

x86 EFI earlyprink was removed with commit 69c1f396 ("efi/x86:
Convert x86 EFI earlyprintk into generic earlycon implementation").

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>