- Deprecate makeFeatureDbFromUCSC(), supportedUCSCFeatureDb*(), and
  UCSCFeatureDbTableSchema() in BioC 3.20. Not a single Bioconductor
  package is using this. Then defunct in BioC 3.21. Once defunct, we
  can get rid of rtracklayer dependency.

- makeTxDbFromUCSC(): Address the CDS non-completeness issue.
  The status of the CDS (complete or not) can be inferred from the
  cds*Stat field (when present) of the UCSC table.

