bloblist: Drop spare value from bloblist record

Drop spare value from bloblist record header.

For now it is still present in the header, with an underscore, so that
tests continue to pass.

Signed-off-by: Simon Glass <sjg@chromium.org>
Co-developed-by: Raymond Mao <raymond.mao@linaro.org>
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>