refactor(test_runner): address the drop_non_drop clippy lint

This `drop` call is intended to cut short the borrow `'a` maintained by
a reference-containing `Mutex<&'a mut _>`, but actually it's unnecessary
because `Mutex`'s contained data (`&'a mut _`) is not subject to [drop
check][1], hence the borrow is allowed to expire before the containing
`Mutex` is dropped.

As for the `drop_non_drop` lint, I would say it's a false-positive
because not every target supported by `std` has a trivial destructor for
`Mutex`. This lint started to trigger only recently because
[rust-lang/rust#95035][2] replaced the `Mutex` implementation with a
futex-based one on Linux.

[1]: https://doc.rust-lang.org/nomicon/dropck.html
[2]: rust-lang/rust#95035

src/r3_test_runner/src/targets/kflash.rs

@@ -207,7 +207,6 @@ impl DebugProbe for KflashDebugProbe {
             })
             .await
             .map_err(RunError::Communication)?;
-            drop(serial_m);
 
             // Program the executable image
             for (i, region) in regions.iter().enumerate() {