Remove outdated use-case description from doc comment
Remove outdated use-case description from doc comment

We stopped advertising binary-wide setting of a label quite a while
ago. This doc comment was missed in the cleanup.

Signed-off-by: beorn7 <a31805f6001ab3b535ea95133fbc4db46e97e8d1@rabenste.in>

m3db/prometheus_client_golang,prometheus/client_golang