MultiOffRamp - remove price update sequence number setter (#1073)

## Motivation
Since merkle root and price updates are bundled together in one Commit
Report, they both share the same `sequenceNumber` that comes from the
`reportContext`. ~~Per OCR3, this `sequenceNumber` is monotonically
increasing, except in cases of config digest updates or issues with the
protocol.~~

~~The current OffRamp logic allows the same sequenceNumber to appear
more than once in a `Transmitted` event, which breaks the OCR3 spec.~~

## Solution
* ~~Enforce `sequenceNumber > latestSequenceNumber`~~ - this is by
design to allow out-of-order execution between a `t + 1` merkle root
report and a `t + 2` price update.
* Remove `setLatestSequenceNumber` function. Price updates can be done
manually in the worst case scenario. The sequence number reset can
already be done via the set OCR config flow.