Greenplum had support for parsing YYYYMMDDHH24MISS timestamps, which
upstream did not.  This is however problematic since it cannot be
parsed unambigously. The following example shows a situation when
the parsing will perform a result which is unlikely to be what the
user was expecting:

  postgres=# set datestyle to 'mdy';
  SET
  postgres=# select '13081205132018'::timestamp;
        timestamp
  ---------------------
   1308-12-05 13:20:18
  (1 row)

The original intent was to aid ETL jobs from Teradata which only
supported this format. This is no longer true according to Teradata
documentation.

This retires the functionality (which was highlighted during the
merge process) aligning the code with upstream, and adds a negative
test for it, the corresponding documentation changes in the release
notes are already done. The existing test for this in qp_misc_jiras
was removed along with a test for a format supported in upstream,
which was already covered by existing suites.
Reviewed-by: NRob Eckhardt <reckhardt@pivotal.io>