Simplify Makefile promu fetch
Simplify Makefile promu fetch

Closes: https://github.com/prometheus/prometheus/issues/3460

GoogleCloudPlatform/prometheus,kangwoo/prometheus,keegancsmith/prometheus,alileza/prometheus,kangwoo/prometheus,amorken/prometheus,Gouthamve/prometheus,prometheus/prometheus,amorken/prometheus,Gouthamve/prometheus,kangwoo/prometheus,kangwoo/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,alileza/prometheus,amorken/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,amorken/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,prometheus/prometheus