Remove zuul._projects

This variable has been transitioned to zuul.projects in the dependent
changes, so is no longer required.

Depends-On: Ib71241a862cf29f6e542dc1d4a2c5cd554d5d7d8
Depends-On: I1408a15d4da274f3fc911f03d5572f4818bc4ebb
Change-Id: I283e66293110aa3bc99acb1cbc6eb3e0cc11f750
3 files changed