fix: remove standalone archiver and PXE start modes

[AztecBot]

Summary

• Removed the standalone aztec start --archiver code path -- the archiver is always embedded in the node, so using --archiver alone now exits with a helpful error message (same pattern as --sequencer)
• Added a handler for aztec start --pxe that similarly errors out instead of silently falling through to "No module specified to start"
• Deleted the dead start_archiver.ts file
• Updated --archiver and --pxe flag descriptions in CLI options and docs to clarify they configure components within a node (use with --node)

The --archiver.* and --pxe.* configuration sub-options remain available for configuring these components when running a full node via --node.

Context

• --archiver as a standalone mode was removed conceptually but the code path and docs were never cleaned up
• --pxe was defined in commander options but never had a handler, causing a confusing "No module specified" error
• Setup guides (running-a-node.md, sequencer-setup.md, running-a-prover.md) correctly use --archiver as a component flag alongside --node and are unaffected

fixes: #22293