Delete `SHARE_BUILD_WITH_JOB_TYPE` feature.

[jonathanmetzman]

It's unclear if we are using it at all and it's implementation requires this horrendous bit of code:

  # Check if this build is dependent on any other custom job. If yes,
  # then fake out our job name for setting up the build.
  old_job_name = ''
  share_build_job_type = environment.get_value('SHARE_BUILD_WITH_JOB_TYPE')
  if share_build_job_type:
    job_name = share_build_job_type
    old_job_name = environment.get_value('JOB_NAME', '')
    environment.set_value('JOB_NAME', job_name)
  else:
    job_name = environment.get_value('JOB_NAME', '')