cmd, les, tests: remove light client code (ethereum#28586)

* cmd, les, tests: remove light client code

This commit removes the light client (LES) code.
Since the merge the light client has been broken and
it is hard to maintain it alongside the normal client.
We decided it would be best to remove it for now and
maybe rework and reintroduce it in the future.

* cmd, eth: remove some more mentions of light mode

* cmd: re-add flags and mark as deprecated

* cmd: warn the user about deprecated flags

* eth: better error message