HeterodyneSearch cleanup (Follow-up from "Move lalapps/src/pulsar/ codes to LALPulsar")

The following discussion from !1904 (merged) should be addressed:

• @matthew-pitkin started a discussion: (+1 comment)

  @karl-wette thanks very much for doing this. I'm happy with the proposed changes, including the lalapps_heterodyne_pulsar and lalapps_pulsar_parameter_estimation_nested executable name changes.

  One thing that could be done in this (although it could wait until a later MR) is a minor clean-up of some stuff in the HeterodyneSearch folder. I'm neither going to maintain or use the HeterodyneCrabPulsar.[c/h] files, so they could just be removed, along with the crab_ephemeris.txt, timingnoise.m and MJD_to_GPS.m files.