Don't bail out of formatting code blocks if lines are >100 chars

[ayazhafiz]

Today rustfmt bails when trying to handle a code block that, after
formatting, has lines >100 chars. The reasoning was that if there were
lines longer than 100 chars it wasn't clear if we failed to format or
that was the intended result, so to be cautious the handler would bail.

However, if formatting the snippet fails the code block formatter will
already have jumped out by the time this line-width check is done:

rustfmt/src/formatting/util.rs

Line 81 in ff57cc6

let mut formatted = format_snippet(&snippet, &config_with_unix_newline)?;

So I am not sure that this check is needed, and removing it broke no
tests except one designed explicitly for it. NB w.r.t. that unit test,
formatting a code like

fn main() {
    let expected = "this_line_is_100_characters_long_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx(x, y, z);";
}

would yield no change, so I believe it is also the expected result.

Closes #4325

[topecongiro]

I am a bit worried about the side effect this PR may introduce, but still, I think the benefit of fixing the #4325 outweighs the potential side effect risk. So LGTM.

[topecongiro]

Thank you 🎉

[ayazhafiz]

I am a bit worried about the side effect this PR may introduce

Yes, I agree. If/when we get an issue because of this change, then we can concretely point and say why this cannot be changed.