The variable "use_hbd" was defined but not used in four places in the
codebase. The warning that it wasn't used was missed by Jenkins
because it only tries one experimental flag at a time, but these
definitions were inside nested experimental flag conditions. Both
CONFIG_WARPED_MOTION and CONFIG_AOM_HIGHBITDEPTH had to be set for the
variables to be defined.

This patch deletes the unused definitions to remove the unused variable
warnings when the AV1 encoder and/or decoder are built with warped
motion and high bitdepth configured.

Change-Id: I9e80523db05a7a19cebe3ec8085a4a1e4b198c52