[bug 1202288] Back out pbr
[bug 1202288] Back out pbr

pbr was a requirement of mock 1.3.0. Since we downgraded mock because
pbr got fussy with vendor in our server environments, we can remove pbr
again.

mozilla/fjord,lgp171188/fjord,lgp171188/fjord,hoosteeno/fjord,lgp171188/fjord,Ritsyy/fjord,Ritsyy/fjord,mozilla/fjord,hoosteeno/fjord,lgp171188/fjord,mozilla/fjord,hoosteeno/fjord,Ritsyy/fjord,Ritsyy/fjord,mozilla/fjord,hoosteeno/fjord