[split-node] Replace starting by default in node service mode with starting a combined mode

[jellonek]

#6618 is introducing subcommands to go-spacemesh: node and smeshing.
node, depending on provided configuration is starting "legacy mode" (a single process with local calls from node service part to smeshing part) or "node service mode".
smeshing starts "smeshing service".

If subcommand was provided - the application is defaulting to node subcommand to keep cmdline backward compatibility.

This issue is about turning off this default fallback, or replacing it with igniting "combined mode" if such will be added in the future (what is still undecided at this time).

That can be done only when CI/all integrations/testnets/devnets will be updated to use node/smeshing subcommands, or - if we will decide to go for separate binaries - to use these separate binaries.