Remove Drop-caused migration-added captures

All of these were added due to insignificant Drop types being present.

compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs

@@ -40,10 +40,7 @@ where
             info!("fully_perform({:?})", self);
         }
 
-        scrape_region_constraints(infcx, || {
-            let _ = &self;
-            (self.closure)(infcx)
-        })
+        scrape_region_constraints(infcx, || (self.closure)(infcx))
     }
 }
 

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -394,7 +394,6 @@ fn report_conflicting_impls(
     // now because the struct_lint methods don't return back the DiagnosticBuilder
     // that's passed in.
     let decorate = |err: LintDiagnosticBuilder<'_>| {
-        let _ = &overlap;
         let msg = format!(
             "conflicting implementations of trait `{}`{}{}",
             overlap.trait_desc,

compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs

@@ -105,7 +105,6 @@ impl ChildrenExt for Children {
 
                 // FIXME: should postpone string formatting until we decide to actually emit.
                 with_no_trimmed_paths(|| {
-                    let _ = &overlap;
                     OverlapError {
                         with_impl: possible_sibling,
                         trait_desc: trait_ref.print_only_trait_path().to_string(),