FancyTabWidget: Remove an old, unnecessary hack

When clicking on a mode button, the emission of `currentChanged` was
delayed by an event. That was supposedly to "improve the responsiveness"
of the mode bar, but I don't see any difference, and _if_ changing the
mode widget was a performance issue, this wouldn't be the right place to
fix or work around it.

The hack creates issues, because it separates the `currentAboutToChange`
and `currentChanged` signals by an event processing.

Amends bd2ba2307b

Change-Id: I53a7aa74fdcfb733a28574837ca9e95fc2e97f0b
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/coreplugin/fancytabwidget.cpp

@@ -196,10 +196,7 @@ void FancyTabBar::mousePressEvent(QMouseEvent *event)
                         emit currentAboutToChange(index);
                         m_currentIndex = index;
                         update();
-                        // update tab bar before showing widget
-                        QMetaObject::invokeMethod(this, [this] {
-                            emit currentChanged(m_currentIndex);
-                        }, Qt::QueuedConnection);
+                        emit currentChanged(m_currentIndex);
                     }
                 }
             }