- LINUX: option to optionally set a sighandler for certain simulators,
  no practical relevance
- LINUX_LIST: not used anywhere
- DEBUG_DLSCH: referenced in the code, but those files have a separate
  (commented) line `#define DEBUG_DLSCH`; probably never used
- LINK_ENB_PDCP_TO_IP_DRIVER, LINK_ENB_PDCP_TO_GTPV1U: commented in
  cmake, although they are referenced. It is likely that code behaves
  unexpected if re-enabled; we therefore remove these options to avoid
  future problems
- Remove some more commented code