`polkadot-omni-node`: re-namings

[kianenigma]

• Rename existing polkadot-parachain to polkadot-omni-node, and the existing polkadot-parachain-lib to polkadot-omni-node-lib.
• Remove any hardcoded chain-spec from polkadot-omni-node
• Then, Create a new polkadot-parachain that is now an instantiation of polkadot-omni-node-lib with system chain specs hardcoded.

A continuation of: #3944

It would be good to do this sooner rather than later to prevent breaking the names any longer.

[serban300]

Performed the renamings as part of #5915

Left to do:

• Publish the polkadot-omni-node binary