Remove block fee, disallow empty blocks.

[afck]

Motivation

If all incoming messages are covered by grants, the chain owner should not have any fees left to pay.

Proposal

Remove the base block fee. Disallow empty blocks.

Test Plan

TBD

Release Plan

• Nothing to do / These changes follow the usual release cycle.

Links

• Part of https://github.com/linera-io/linera-protocol/issues/3841.
• reviewer checklist