Remove StartSpan function from `tele` package

[iand]

Migrated from libp2p/go-libp2p-kad-dht#927

We should remove this function: https://github.com/libp2p/go-libp2p-kad-dht/blob/e86381e824dd2797cf931f14070292fe763a0c93/v2/tele/tele.go#L133

Instead we should plumb a tracer through to the pool/probe/etc.

Context: libp2p/go-libp2p-kad-dht#924 (comment)