time: remove outdated explicitly drop call of Mutex (#7326)

This drop was firstly introduced by [#3289],
and the next line invokes `panic!`.

In [#5434], the original `panic!` was replaced
with `return Err`, so dropping it explicitly
is no longer necessary.

[#3289]: https://github.com/tokio-rs/tokio/pull/3289
[#5434]: https://github.com/tokio-rs/tokio/pull/5434

tokio/src/time/clock.rs

@@ -251,7 +251,6 @@ cfg_test_util! {
             let mut inner = self.inner.lock();
 
             if !inner.enable_pausing {
-                drop(inner); // avoid poisoning the lock
                 return Err("`time::pause()` requires the `current_thread` Tokio runtime. \
                         This is the default Runtime used by `#[tokio::test].");
             }