Don't access static functions/fields via instance

Courtesy of readability-static-accessed-through-instance

Amends: b2a766a79a

Round #2: This time done with Qt Creator's Analyzer, which
found other occurences than run-clang-tidy.py

Change-Id: I479e280c7abcf2d24baccbb0af69ae4bda05198e
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/coreplugin/imode.cpp

@@ -122,7 +122,7 @@ namespace Core {
 */
 IMode::IMode(QObject *parent) : IContext(parent)
 {
-    ModeManager::instance()->addMode(this);
+    ModeManager::addMode(this);
 }
 
 /*!
@@ -130,7 +130,7 @@ IMode::IMode(QObject *parent) : IContext(parent)
 */
 IMode::~IMode()
 {
-    ModeManager::instance()->removeMode(this);
+    ModeManager::removeMode(this);
 }
 
 void IMode::setEnabled(bool enabled)