Remove Boost conditional compilation.

Boost is currently at version 106200 in the depends subsystem
so it is safe to remove code the refers to earlier versions.

This closes #2445

src/util.cpp

@@ -911,10 +911,6 @@ std::string LicenseInfo()
 
 int GetNumCores()
 {
-#if BOOST_VERSION >= 105600
     return boost::thread::physical_concurrency();
-#else // Must fall back to hardware_concurrency, which unfortunately counts virtual cores
-    return boost::thread::hardware_concurrency();
-#endif
 }