fs: use async fn instead of custom futures (#1381)

Also update all the doc examples.

tokio-fs/src/remove_file.rs

@@ -1,10 +1,7 @@
-use std::fs;
-use std::future::Future;
+use crate::asyncify;
+
 use std::io;
 use std::path::Path;
-use std::pin::Pin;
-use std::task::Context;
-use std::task::Poll;
 
 /// Removes a file from the filesystem.
 ///
@@ -15,36 +12,6 @@ use std::task::Poll;
 /// This is an async version of [`std::fs::remove_file`][std]
 ///
 /// [std]: https://doc.rust-lang.org/std/fs/fn.remove_file.html
-pub fn remove_file<P: AsRef<Path>>(path: P) -> RemoveFileFuture<P> {
-    RemoveFileFuture::new(path)
-}
-
-/// Future returned by `remove_file`.
-#[derive(Debug)]
-#[must_use = "futures do nothing unless you `.await` or poll them"]
-pub struct RemoveFileFuture<P>
-where
-    P: AsRef<Path>,
-{
-    path: P,
-}
-
-impl<P> RemoveFileFuture<P>
-where
-    P: AsRef<Path>,
-{
-    fn new(path: P) -> RemoveFileFuture<P> {
-        RemoveFileFuture { path }
-    }
-}
-
-impl<P> Future for RemoveFileFuture<P>
-where
-    P: AsRef<Path>,
-{
-    type Output = io::Result<()>;
-
-    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
-        crate::blocking_io(|| fs::remove_file(&self.path))
-    }
+pub async fn remove_file<P: AsRef<Path>>(path: P) -> io::Result<()> {
+    asyncify(|| std::fs::remove_file(&path)).await
 }