Cleanup/remove deprecate chains cmd

[adityavshinde]

Changes

fixes: #2751

• Removed pkg/cmd/chain/chain.go
• Removed pkg/cmd/chain/signature.go
• Removed pkg/cmd/chain/payload.go
• Removed docs/cmd/tkn_chain.md
• Removed docs/cmd/tkn_chain_signature.md
• Removed docs/cmd/tkn_chain_payload.md
• Modified pkg/cmd/root.go
• Modified pkg/cmd/testdata/TestPluginList.golden

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[waveywaves]

@adityavshinde the CI isn't green here

[waveywaves]

cc @vdemeester