Remove useless warning

tests/ui/lifetimes.rs

@@ -145,7 +145,7 @@ impl Test {
 
 trait LintContext<'a> {}
 
-fn f<'a, T: LintContext<'a>>(cx: &T) {}
+fn f<'a, T: LintContext<'a>>(_: &T) {}
 
 fn test<'a>(x: &'a [u8]) -> u8 {
     let y: &'a u8 = &x[5];

tests/ui/lifetimes.stderr

@@ -97,13 +97,5 @@ error: explicit lifetimes given in parameter types where they could be elided
 132 | fn trait_bound_bug<'a, T: WithLifetime<'a>>() { unimplemented!() }
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-warning: unused variable: `cx`
-   --> $DIR/lifetimes.rs:148:30
-    |
-148 | fn f<'a, T: LintContext<'a>>(cx: &T) {}
-    |                              ^^
-    |
-    = note: #[warn(unused_variables)] on by default
-
 error: aborting due to 15 previous errors