Remove all observationForCount/All/First methods

[groue]

This pull request leaves ValueObservation.tracking(_:) as the unique way to create a ValueObservation, by removing all remaining convenience methods.

// BEFORE: GRDB 4
let observation = request.observationForCount()
let observation = request.observationForFirst()
let observation = request.observationForAll()
let observation = ValueObservation.tracking(request, fetch: { db in ... })

// NEW: GRDB 5
let observation = ValueObservation.tracking(request.fetchCount)
let observation = ValueObservation.tracking(request.fetchOne)
let observation = ValueObservation.tracking(request.fetchAll)
let observation = ValueObservation.tracking { db in ... }