Remove the at least one validator condition in config.yml validation

[lumtis]

The current to validate a config.yml fails if no validator is defined in the config.

I propose removing this check.

validators is used for the serve command for local experimentation of the chain.
However, config.yml has more usage than just for the serve command.
The config can contain build instructions that can be used for the build command to build the binary.
The network commands also read the config for build instruction without reading the validators.
The genesis config option for chain publish must contain no validator (although this one follow another validation routine)

Validator check should not be part of the verification for a minimal valid config. This check should specifically belong to the serve command, or in general a feature specific check should belong to the associated command