Remove incorrect expect(unused) annotation (#1789)

Both #1780 and
#1778 fixed the
`snapshot_exists` function only being used on illumos. Unfortunately,
they fixed it in mutually incompatible ways: #1780 added a trivial test
to exercise it, while #1778 added an `expect(unused)` annotation. The
combination of these two produces a warning on non-illumos machines.

Author: mkeeter

integration_tests/src/lib.rs

@@ -714,7 +714,6 @@ mod integration_tests {
             self.downstairs3.address()
         }
 
-        #[cfg_attr(not(target_os = "illumos"), expect(unused))]
         pub fn snapshot_exists(&self, snapshot_name: &str) -> Result<bool> {
             Ok(self.downstairs1.snapshot_exists(snapshot_name)?
                 && self.downstairs2.snapshot_exists(snapshot_name)?