Make variable manager static.

This makes it follow our preferred singleton pattern.

Change-Id: I230e5ac5ef7f156da7123f7efe3a49bcb6a20669
Reviewed-by: hjk <hjk121@nokiamail.com>

src/plugins/coreplugin/dialogs/externaltoolconfig.cpp

@@ -610,5 +610,5 @@ void ExternalToolConfig::addCategory()
 void ExternalToolConfig::updateEffectiveArguments()
 {
     ui->arguments->setToolTip(Utils::QtcProcess::expandMacros(ui->arguments->text(),
-            Core::VariableManager::instance()->macroExpander()));
+            Core::VariableManager::macroExpander()));
 }