Fix clippy::needless_lifetimes warning

Clippy nightly started warning e.g.:

```
warning: the following explicit lifetimes could be elided: 'conn
  --> x11rb-async/src/cookie.rs:68:6
   |
68 | impl<'conn, C: RequestConnection + ?Sized> Drop for VoidCookie<'conn, C> {
   |      ^^^^^                                                     ^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
   |
68 - impl<'conn, C: RequestConnection + ?Sized> Drop for VoidCookie<'conn, C> {
68 + impl<C: RequestConnection + ?Sized> Drop for VoidCookie<'_, C> {
   |
```

Signed-off-by: Uli Schlachter <psychon@znc.in>

x11rb-async/src/cookie.rs

@@ -65,7 +65,7 @@ impl<'conn, C: Connection + ?Sized> VoidCookie<'conn, C> {
     }
 }
 
-impl<'conn, C: RequestConnection + ?Sized> Drop for VoidCookie<'conn, C> {
+impl<C: RequestConnection + ?Sized> Drop for VoidCookie<'_, C> {
     fn drop(&mut self) {
         self.conn.discard_reply(
             self.sequence,
@@ -94,7 +94,7 @@ impl<'a, C: RequestConnection + ?Sized> RawCookie<'a, C> {
     }
 }
 
-impl<'a, C: RequestConnection + ?Sized> Drop for RawCookie<'a, C> {
+impl<C: RequestConnection + ?Sized> Drop for RawCookie<'_, C> {
     fn drop(&mut self) {
         self.conn.discard_reply(
             self.sequence,