Compile fix: remove unused variable bool isEmpty

The local variable isEmpty triggers a compile warning with GCC:
variable ‘isEmpty’ set but not used [-Werror=unused-but-set-variable],
so remove the variable assignment and declaration.

Error is triggered on FreeBSD and NetBSD with gcc.

Change-Id: I37bdb3408ad69093708f2d4bdb04392da66e04e5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qfilesystemwatcher_kqueue.cpp

@@ -166,7 +166,6 @@ QStringList QKqueueFileSystemWatcherEngine::removePaths(const QStringList &paths
                                                         QStringList *files,
                                                         QStringList *directories)
 {
-    bool isEmpty;
     QStringList p = paths;
     if (pathToID.isEmpty())
         return p;
@@ -187,7 +186,6 @@ QStringList QKqueueFileSystemWatcherEngine::removePaths(const QStringList &paths
         else
             files->removeAll(path);
     }
-    isEmpty = pathToID.isEmpty();
 
     return p;
 }