libnet/config: rm config_freebsd.go

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

daemon/libnetwork/config/config_freebsd.go

@@ -1,8 +0,0 @@
-package config
-
-// FIXME(thaJeztah): ExecRoot is only used for Controller.startExternalKeyListener(), but "libnetwork-setkey" is only implemented on Linux.
-func optionExecRoot(execRoot string) Option {
-	return func(c *Config) {
-		c.ExecRoot = execRoot
-	}
-}