Remove extra mutex unlocking in ConcurrentQueue.h

As discussed in https://github.com/DennisOSRM/Project-OSRM/pull/998 , unlocking the mutex is performed on destruction. Second unlocking gives an assertion (for debug version) for Windows and FreeBSD 10.
Merge branch 'patch-6' of https://github.com/alex85k/Project-OSRM into alex85k-patch-6
Merge branch 'alex85k-patch-6' into develop