ExtensionSystem: Stop reworking initialization order system

This reverts b91f234c7d and some changes on top.

With the static-in-function approach the dependency seem to be
more easier to solve than with the the central mechanism here.

Change-Id: I65be3ced847b31c25637280376f2fe30003f0527
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/extensionsystem/pluginspec.cpp

@@ -1124,7 +1124,6 @@ bool PluginSpecPrivate::initializePlugin()
         hasError = true;
         return false;
     }
-    plugin->tryCreateObjects();
     state = PluginSpec::Initialized;
     return true;
 }
@@ -1151,7 +1150,6 @@ bool PluginSpecPrivate::initializeExtensions()
         return false;
     }
     plugin->extensionsInitialized();
-    plugin->tryCreateObjects();
     state = PluginSpec::Running;
     return true;
 }
@@ -1172,7 +1170,6 @@ bool PluginSpecPrivate::delayedInitialize()
         return false;
     }
     const bool res =  plugin->delayedInitialize();
-    plugin->tryCreateObjects();
     return res;
 }