Drop X11 core fonts code from startkde/startplasma

Summary:
Qt and GTK based applications have switched to using fontconfig for fonts
exclusively, so this code is mostly unused.

As for the remaining legacy X11 applications:
- mkfontdir only supports bitmap (e.g. PCF, BDF) fonts. For e.g.
  truetype fonts, a mkfontscale call would be required. Lack of complaints
  hints this is not used at all.
- the system dirs (sys_odir, sys_fdir) use KDEDIR(S), which is deprecated.
  The dirs do not exist, thus the fontpath is unchanged.
- system dirs are typically allready included in the XServer font path

TLDR: (scalable) fonts in user dirs are not picked up due to missing
mkfontscale calls, and system dirs are already included.

This removes the dependency on xset and mkfontdir. The latter is important,
as e.g. on openSUSE the availability of mkfontdir triggers costly, extra
setup steps for X Core fonts on package installation.

Reviewers: #plasma, fvogt, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16530