fix clippy::needless_lifetimes

warning: explicit lifetimes given in parameter types where they could be elided (or replaced with `'_` if needed by type declaration)
   --> tarpc/src/rpc/server/filter.rs:127:5
    |
127 |     fn channel<'a>(self: Pin<&'a mut Self>) -> Pin<&'a mut C> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(clippy::needless_lifetimes)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes

warning: 1 warning emitted

tarpc/src/rpc/server/filter.rs

@@ -124,7 +124,7 @@ impl<C, K> TrackedChannel<C, K> {
     }
 
     /// Returns the pinned inner channel.
-    fn channel<'a>(self: Pin<&'a mut Self>) -> Pin<&'a mut C> {
+    fn channel(self: Pin<&mut Self>) -> Pin<&mut C> {
         self.project().inner
     }
 }