Consider removing invalid span workaround form join_spans

[mystor]

In the internal join_spans function, there is some old code which filters out invalid tokens from the input stream by debug-printing the Span object, and inspecting the output.

quote/src/spanned.rs

Lines 21 to 32 in 7d5d360

	let mut iter = tokens.into_iter().filter_map(|tt| {
	// FIXME: This shouldn't be required, since optimally spans should
	// never be invalid. This filter_map can probably be removed when
	// https://github.com/rust-lang/rust/issues/43081 is resolved.
	let span = tt.span();
	let debug = format!("{:?}", span);
	if debug.ends_with("bytes(0..0)") {
	None
	} else {
	Some(span)
	}
	});

Given that rust-lang/rust#43081 appears to have been fixed for a bit over a year (rust-lang/rust#43081 (comment)), perhaps this code could be disabled on versions with the fix (appears to be rustc 1.53+)?