Delete MustLoadFromString, as nobody needs it.
Delete MustLoadFromString, as nobody needs it.

Change-Id: Ib95c316857ffdd36d973c0e8701967de99d78b13

asbjornenge/client_golang,telendt/client_golang,m3db/prometheus_client_golang,KevinPike/client_golang,fabxc/client_golang,prometheus/client_golang,mattkanwisher/client_golang,jimmidyson/prometheus-client_golang,stapelberg/client_golang