Allow flycheck process to exit gracefully

Assuming it isn't cancelled. Closes rust-lang#17902.

The only place CommandHandle::join is used is when the flycheck command
finishes, so this commit changes the behavior of the method itself.

src/tools/rust-analyzer/crates/rust-analyzer/src/command.rs

@@ -148,7 +148,6 @@ impl<T: ParseFromLine> CommandHandle<T> {
     }
 
     pub(crate) fn join(mut self) -> io::Result<()> {
-        let _ = self.child.0.kill();
         let exit_status = self.child.0.wait()?;
         let (read_at_least_one_message, error) = self.thread.join()?;
         if read_at_least_one_message || exit_status.success() {