Revert "Core/Spells: Fix a crash on checking school immunity added to unit manually (not by aura)"

This reverts commit 07badd7385.

src/server/game/Server/WorldSocket.cpp

@@ -893,7 +893,7 @@ int WorldSocket::HandleAuthSession (WorldPacket& recvPacket)
     //! Negative mutetime indicates amount of seconds to be muted effective on next login - which is now.
     if (mutetime < 0)
     {
-        mutetime = time(NULL) + -mutetime;
+        mutetime = time(NULL) + llabs(mutetime);
         LoginDatabase.PExecute("UPDATE account SET mutetime = " SI64FMTD " WHERE id = '%u'", mutetime, id);
     }