[builder] usage of inactive "version" keyword in config.yaml

From https://github.com/googlefonts/gftools/blob/main/Lib/gftools/builder/schema.py, there is a key named version available in the config.yaml. However, this version record is not used when building the font files (not for version string nor head.revision).

The commit for the line points to https://github.com/googlefonts/gftools/issues/394 and https://github.com/googlefonts/gftools/issues/380 but the ability seem to be lost now.