ExtensionSystem: Introduce alias PluginSpecs

Change-Id: Ic7b87246cf91328c4291bbce5760a45dfa0b14e5
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/extensionmanager/extensionsbrowser.cpp

@@ -37,7 +37,6 @@ namespace ExtensionManager::Internal {
 
 Q_LOGGING_CATEGORY(browserLog, "qtc.extensionmanager.browser", QtWarningMsg)
 
-using PluginSpecList = QList<const PluginSpec *>;
 using Tags = QStringList;
 
 constexpr QSize itemSize = {330, 86};