Drop the ineffective logo height and width configurations

The $LogoImageHeight and $LogoImageWidth configurations were only output
on the <img> tag if the stock BPS logo was in use.  Additionally,
however, those values were ignored by the browser, because every RT
style sets:

    div#logo a img {
         height: 100% !important;
         width: auto !important;
    }

...which overrides explicit height="..." and width=".." attributes.

Remove the configuration options which thus do not actually control
anything.

Fixes: I#26827
Merge branch '4.2/drop-logo-size' into 4.2-trunk