FN needless_return: "return" at end of void-function

If we have a function that does not return a type and has `return` as last statement, we could suggest that that return could be omitted:
````rust
pub fn a() {
	println!("blurb");
	return; // unneeded
}

pub fn b() {
	println!("blarb");
	return // unneeded
}
````
=>

````rust
pub fn a() {
	println!("blurb");
}

pub fn b() {
	println!("blarb");
}
````