xtensa: Remove unused early_read_config_byte() et al declarations

early_read_config_byte() and similar are declared but never defined.
Remove the unused declarations.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Message-Id: <20220121210258.1152803-1-helgaas@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>