BibField: no caching of calculated fields

* Introduces no caching of calculated fields as a workaround for issues
  related to cache invalidation.  (E.g. when the citation indexer runs
  and updates citation counts, the recjson cache is not invalidated,
  either directly or via some time expiry mechanism, so that the number
  of citations would remain invalid until some process would touch
  record metadata.)

* NOTE The cache invalidation issue will be properly addressed later.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
Co-authored-by: Esteban J. G. Gabancho <egabancho@gmail.com>