Fix the clippy assert!(false) lint

https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants

While I don't feel it's valid, fixing it lets us act on the other, more
useful, lints.

tests/sqlparser_ansi.rs

@@ -14,6 +14,6 @@ fn parse_simple_select() {
         }) => {
             assert_eq!(3, projection.len());
         }
-        _ => assert!(false),
+        _ => unreachable!(),
     }
 }