Revert "Implement translation of union type from ast."

This reverts commit 3ef5585b035a5fc6716682c33efea919774dfae6.

Reason for revert:

Breaks compile (e.g., http://build.chromium.org/p/client.mojo/builders/Mojo%20Linux%20%28dbg%29/builds/939/steps/mojob%20build/logs/stdio).

TBR=azani

Review URL: https://codereview.chromium.org/801693002