ExtensionSystem: Remove the IPlugin back pointer to pluginspec

The price of having to loop in two places seems small compared
to cleaner relations between the classes.

There's a new hack in the helpmanager to make sure we aren't
looping to often. The hack wouldn't be needed if the (odd(?))
check there weren't there.

Change-Id: Ifed50213b2de8feedfb45c185808d163c00c19ca
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/extensionsystem/pluginspec.cpp

@@ -5,7 +5,6 @@
 
 #include "extensionsystemtr.h"
 #include "iplugin.h"
-#include "iplugin_p.h"
 #include "pluginmanager.h"
 #include "pluginspec_p.h"
 
@@ -1090,7 +1089,6 @@ bool PluginSpecPrivate::loadLibrary()
     }
     state = PluginSpec::Loaded;
     plugin = pluginObject;
-    plugin->d->pluginSpec = q;
     return true;
 }