unset_first(conf, type, opt) {
		return this.set_first(conf, type, opt, null);
	}
Removes the given option within the first found section of the given
configuration matching the specified type or within the first section
of the entire config when no type has is specified.

This function is a convenience wrapper around
`uci.set_first(config, type, option, null)`.

@param {string} conf
The name of the configuration to set the option value in.

@param {string} [type]
The type of the first section to find. If it is `null`, the first
section of the entire config is written to, otherwise the first
section matching the given type is used.

@param {string} opt
The option name to set the value for.