Fix subspace-node purge-chain command

[hazyone]

This PR fixes problem #1215 for thesubspace-node.

As I realized, clap-rs made implicit ArgGroup creation, and PurgeChainCmd contains the same named struct in it:

pub struct PurgeChainCmd {
    /// The base struct of the purge-chain command.
    #[clap(flatten)]
    pub base: sc_cli::PurgeChainCmd,
}

So, as mentioned in the error description, we have to use #[group(skip)] to avoid this behavior.

After fixing the command works as desired:

 > ./subspace-node purge-chain -d nodee/ --chain gemini-3c

nodee/system/chains/subspace_gemini_3c_system_domain/paritydb/full
nodee/chains/subspace_gemini_3c/paritydb/full
Are you sure to remove? [y/N]: y
"nodee/system/chains/subspace_gemini_3c_system_domain/paritydb/full" did not exist.
"nodee/chains/subspace_gemini_3c/paritydb/full" removed.

...
> ls nodee/chains/subspace_gemini_3c/paritydb/full
ls: nodee/chains/subspace_gemini_3c/paritydb/full: No such file or directory

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[nazar-pc]

Makes sense to me, thanks!