Remove Python 3.6 installation from Dockerfile.package-cu102

[leandron]

• Removing it due to deprecation and not being present in the latest version on the base image

Base image information:

pytorch/manylinux-cuda102           latest                      2146d9ccbb6e

cc @areusch @tqchen @Mousius

[leandron]

Expected to fail until https://github.com/tlc-pack/tlcpack/pull/92 is merged.

[areusch]

do we need to wait til CI updates to 3.7 for this?

[leandron]

do we need to wait til CI updates to 3.7 for this?

No, because the production image already shipped without 3.6.