Rollup merge of #105243 - RalfJung:no-op-let, r=Mark-Simulacrum

remove no-op 'let _ = '

Also see the discussion at rust-lang/rust#93563 (comment).

I don't know why these `Drop` implementations exist to begin with, given that their body does literally nothing, but did not want to change that. (It might affect dropck.)

Cc `````@ibraheemdev````` `````@Amanieu`````

std/src/sync/mpsc/mod.rs

@@ -629,9 +629,7 @@ impl<T> Clone for Sender<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Drop for Sender<T> {
-    fn drop(&mut self) {
-        let _ = self.inner;
-    }
+    fn drop(&mut self) {}
 }
 
 #[stable(feature = "mpsc_debug", since = "1.8.0")]
@@ -751,9 +749,7 @@ impl<T> Clone for SyncSender<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Drop for SyncSender<T> {
-    fn drop(&mut self) {
-        let _ = self.inner;
-    }
+    fn drop(&mut self) {}
 }
 
 #[stable(feature = "mpsc_debug", since = "1.8.0")]
@@ -1094,9 +1090,7 @@ impl<T> IntoIterator for Receiver<T> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Drop for Receiver<T> {
-    fn drop(&mut self) {
-        let _ = self.inner;
-    }
+    fn drop(&mut self) {}
 }
 
 #[stable(feature = "mpsc_debug", since = "1.8.0")]