Remove added compatibility layer for QFontMetrics.width deprecation warnings when we drop support for old versions of pyqt5

In #1315 we added a compatibility layer to avoid a deprecation warning when calling QFontMetrics.width(). Namely we added a variation of the following code
```
if QtCore.__version_info__ >= (5, 11):
_____ = QFontMetrics.horizontalAdvance(_____)
else:
_____ = QFontMetrics.width(_____)
```

Once we drop support for older versions of PyQt5, this logic can be removed and we can simply call .horizontalAdvance()