Delete Python3.6 image generation job

Once Silkaj v0.9 and DuniterPy v1.0 support is behind us.

Silkaj v0.10 and DuniterPy v1.1 are over-seeding their support.

Copies of 3.6 image can stay in the registry.

Edited by Moul
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information