rustfmt doesn't always remove unecessary where clause

[kennykerr]

Given:

fn test() where {}

rustfmt will correctly produce:

fn test() {}

But given:

fn test() -> () where {}

rustfmt will fail to remove the unecessary where clause.

[ytmimi]

Duplicate of #4649.

[riverar]

@ytmimi Not sure that's an appropriate issue to dupe on. That issue is concerned with incorrect removal of the where clause. This issue is concerned with incorrect preservation of the where clause.