Consider removing invalid span workaround form `join_spans`

[Nika Layzell]

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+)?