[master] remove conflicting and unused 'using' directive

This should fix the issue reported by the solaris-gcc builder

src/lib/dhcp/tests/alloc_engine_unittest.cc

@@ -30,7 +30,6 @@ using namespace isc;
 using namespace isc::asiolink;
 using namespace isc::dhcp;
 using namespace isc::dhcp::test; // Memfile_LeaseMgr
-using namespace boost;
 
 namespace {