Do not override Heading pixel size in BreadCrumbControl

This fixes the inconsistency found in title sizes between QML KCMs and
QWidget KCMs.

Both KCM titles use the same logic to calculate the title font.pointSize
depending on the heading level:
- `Heading.qml:headerPointSize()` in Kirigami
- `ktitlewidget.cpp:textStyleSheet()` in KConfigWidgets

But setting externally the `Heading.pixelSize` depending on height was
overriding this calculation.

BUG: 404396
FIXED-IN: 5.74
MR: frameworks/kirigami!73