Fix unnecessary lifetime bound

This fixes nightly build

.gitattributes

@@ -1,4 +1,4 @@
 *.rs text eol=lf
 *.lock text eol=lf
 *.txt text eol=lf
-*.toml text eol=lf
+*.toml text eol=lf

src/diskio/threaded.rs

@@ -208,7 +208,7 @@ impl<T: Iterator<Item = Task>> Iterator for JoinIterator<T> {
     }
 }
 
-struct SubmitIterator<'a, 'b: 'a> {
+struct SubmitIterator<'a, 'b> {
     executor: &'a mut Threaded<'b>,
     item: Cell<Task>,
 }