Remove fish_mutex_t wrapper around std::mutex

@ridiculousfish had introduced this in 3a45cad12e
to work around an issue with Coverity Scan where it couldn't tell the
mutex was correctly locked, but even with the `fish_mutex_t` hack, it
still emits the same warnings, so there's no pointing in keeping it.

src/complete.cpp

@@ -1253,9 +1253,7 @@ bool completer_t::try_complete_user(const wcstring &str) {
     bool result = false;
     size_t name_len = str.length() - 1;
 
-    // We don't bother with the thread-safe `getpwent_r()` variant because this is the sole place
-    // where we call getpwent().
-    static fish_mutex_t lock;
+    static std::mutex lock;
     scoped_lock locker(lock);
     setpwent();
     // cppcheck-suppress getpwentCalled