Use a cleaner initialization of `separatorByteSlice`
Use a cleaner initialization of `separatorByteSlice`

The `const separatorByte` wasn't used anymore actually. In `vec.go`,
we were using `model.SeparatorByte`, which is better anyway. So remove
the unused constant and initialize `separatorByteSlice` with
`model.SeparatorByte`, too.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>

prometheus/client_golang,m3db/prometheus_client_golang