Remove old subscription code and other fixes

- removed old subscription related dummy code.
- updated cps version to 3.4.6 to use the latest released version
- added six dependency for the RTD fix
- Removing the CloudEventMapper for now. Can be brought back when
  somebody uses it
- added missing test for InvalidDatastoreException which was failing on
  coverage check locally

Issue-ID: CPS-2149
Change-Id: I60e9036398fea82bb0162ab5524a1cc45dedb611
Signed-off-by: mpriyank <priyank.maheshwari@est.tech>
9 files changed