remove(cmd): Remove `zebrad download` command, because it no longer does anything #73

Job	Run time
Versioning	0s
Build CD Docker / Build images	5m 20s
Test CD testnet Docker config file / Test testnet-conf in Docker	1m 37s
Test CD custom Docker config file / Test custom-conf in Docker	22s
Test CD default Docker config file / Test default-conf in Docker	1m 24s
Deploy single ${{ inputs.network }} instance	0s
Deploy ${{ matrix.network }} nodes	0s
Open or update issues for release failures	0s
	8m 43s

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