Simplify examples by removing server-set timestamps

[amunra]

Our examples (https://github.com/questdb/py-questdb-client/tree/main/examples) document usage of ingestion without the at parameter for sender.dataframe and sender.row.

This makes:

• The examples too lengthy.
• Promote a feature we'd rather see fewer users use.

Any other clean-up to the examples would be most welcome.