remove logging for missing SecureConfig (#238)

In #207 we introduced a warning if the `SecureConfig` is unset. For Nomad and
other applications that have "internal" go-plugin consumers where the
application runs itself as a plugin, this causes spurious warn-level logs. For
Nomad in particular this means every task driver and logmon invocation emits the
log, which is our primary operation.

Setting a `SecureConfig` isn't required, and if a given go-plugin consumer
requires it to be, this seems more like a programming error than a runtime
error. Remove the log line.

client.go

@@ -568,9 +568,7 @@ func (c *Client) Start() (addr net.Addr, err error) {
 		return nil, err
 	}
 
-	if c.config.SecureConfig == nil {
-		c.logger.Warn("plugin configured with a nil SecureConfig")
-	} else {
+	if c.config.SecureConfig != nil {
 		if ok, err := c.config.SecureConfig.Check(cmd.Path); err != nil {
 			return nil, fmt.Errorf("error verifying checksum: %s", err)
 		} else if !ok {