feat: remove warning when the config file is explicitly stdin

[ldez]

Fixes #3739

[bombsimon]

I know this change only removes the warning but if we want to support this, would it make sense to detect content on stdin and automatically use that instead of having to pipe stdin to -c/--config? I was thinking something like

stat, _ := os.Stdin.Stat()
if (stat.Mode() & os.ModeCharDevice) == 0 {
	// Got data on stdin
} else {
	// Use flag or default config
}

Approving this either way since a change like that probably deserves its own PR.

[ldez]

I know this change only removes the warning but if we want to support this, would it make sense to detect content on stdin and automatically use that instead of having to pipe stdin to -c/--config? I was thinking something like

stat, _ := os.Stdin.Stat()
if (stat.Mode() & os.ModeCharDevice) == 0 {
	// Got data on stdin
} else {
	// Use flag or default config
}

I already think about that but this only detects if the command is piped and not if the configuration comes from stdin.
A pipe can be used in different contexts and not only for the configuration file.