just remove clearing password in dtor which is only called on exit

solves the bug found by apprb

src/core.cpp

@@ -90,9 +90,6 @@ Core::Core(Camera* cam, QThread *CoreThread, QString loadPath) :
 
 Core::~Core()
 {
-    clearPassword(Core::ptMain);
-    clearPassword(Core::ptHistory);
-
     if (tox)
     {
         toxav_kill(toxav);