zuul: remove push_change_refs setting

I have upgraded Zuul and the new version no more supports the
push_change_refs settings (was meant to push Zuul references back to
Gerrit which we never used).

Can be merged at anytime.

Bug: 63994
Change-Id: Ia6c252db82945f471da7d0d27e015ede1c7abf4f
Merge "zuul: remove push_change_refs setting" into production