ARROW-6873: [Python] Remove stale CColumn references
ARROW-6873: [Python] Remove stale CColumn references

Closes #5646 from xhochy/ARROW-6873 and squashes the following commits:

bcb6f7871 <Uwe L. Korn> ARROW-6873:  Remove stale CColumn references

Authored-by: Uwe L. Korn <5f6f0a63dee51498bdd8036770b3bad30ce7a5ac@quantco.com>
Signed-off-by: Antoine Pitrou <8ffbbac03241e3002f35c2a1f358cd318666d6e4@free.fr>

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