Remove braces from most E0505 examples

The solution which uses braces to release the borrow
before it is moved is only required to satisfy the 2015
edition borrow checker.

All other examples give the expected results for both
2015 and 2018 editions.

src/librustc_mir/diagnostics.rs

@@ -1551,11 +1551,9 @@ fn eat(val: Value) {}
 
 fn main() {
     let x = Value{};
-    {
-        let _ref_to_val: &Value = &x;
-        eat(x);
-        borrow(_ref_to_val);
-    }
+    let _ref_to_val: &Value = &x;
+    eat(x);
+    borrow(_ref_to_val);
 }
 ```
 
@@ -1579,11 +1577,9 @@ fn eat(val: &Value) {}
 
 fn main() {
     let x = Value{};
-    {
-        let _ref_to_val: &Value = &x;
-        eat(&x); // pass by reference, if it's possible
-        borrow(_ref_to_val);
-    }
+    let _ref_to_val: &Value = &x;
+    eat(&x); // pass by reference, if it's possible
+    borrow(_ref_to_val);
 }
 ```
 
@@ -1618,11 +1614,9 @@ fn eat(val: Value) {}
 
 fn main() {
     let x = Value{};
-    {
-        let _ref_to_val: &Value = &x;
-        eat(x); // it will be copied here.
-        borrow(_ref_to_val);
-    }
+    let _ref_to_val: &Value = &x;
+    eat(x); // it will be copied here.
+    borrow(_ref_to_val);
 }
 ```