Remove the font-size declarations from the CSS, rely on the default
sizes for each element. This solves two issues:

  - The user-configured font size (both in Assistant and Qt Creator
    Help) is correctly taken into use. Previously, adjusting the
    font size resulted in resizing fonts only for some of the text
    elements.

  - QTextBrowser backend seems to assume a DPI of 96 for all
    platforms, and font-sizes in 'pt' units are scaled accordingly.
    However, on OS X this resulted in font sizes too small to read.

Change-Id: Iaab3ad07e387912ad7391df982d4bfe7047f0b20
Task-number: QTBUG-51709
Task-number: QTBUG-51885
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>