refactor: remove straggling boost::mutex usage

After the merge of #18710, the linter is warning:
```bash
A new Boost dependency in the form of "boost/thread/mutex.hpp" appears to have been introduced:
src/sync.cpp:#include <boost/thread/mutex.hpp>
src/test/sync_tests.cpp:#include <boost/thread/mutex.hpp>

^---- failure generated from test/lint/lint-includes.sh
```

the interim #19337 was merged, which introduced more `boost::mutex` usage.

Given we no longer use `boost::mutex`, just remove the double lock test
and remaining includes.

src/test/sync_tests.cpp

@@ -6,7 +6,6 @@
 #include <test/util/setup_common.h>
 
 #include <boost/test/unit_test.hpp>
-#include <boost/thread/mutex.hpp>
 
 #include <mutex>
 
@@ -110,11 +109,6 @@ BOOST_AUTO_TEST_CASE(double_lock_mutex)
     TestDoubleLock<Mutex>(true /* should throw */);
 }
 
-BOOST_AUTO_TEST_CASE(double_lock_boost_mutex)
-{
-    TestDoubleLock<boost::mutex>(true /* should throw */);
-}
-
 BOOST_AUTO_TEST_CASE(double_lock_recursive_mutex)
 {
     TestDoubleLock<RecursiveMutex>(false /* should not throw */);