Remove old reference to basicMetricVec
Remove old reference to basicMetricVec

Contrary to the code comment, I see no `basicMetricVec` implementation.
Grep'ing this project shows this is the only reference. So I suspect
it's an outdated comment and can be removed to minimize confusion.

I'm unclear whether other parts of that comment are also incorrect and
need updating.

Signed-off-by: Jeff Widman <f3e731dfa293c7a83119d8aacfa41b5d2d780be9@jeffwidman.com>

prometheus/client_golang,m3db/prometheus_client_golang