appimage: Do not bundle libfontconfig

Summary:
The bundled libfontconfig seems to cause problems when the target system uses
a newer font config format. See discussion here:
  https://mail.kde.org/pipermail/kdevelop/2018-October/019726.html

It seems to be a common problem (as seen in bug reports of other AppImage
projects).

It's also blacklisted in linuxdeployqt:
  https://github.com/AppImage/AppImages/blob/master/excludelist

For me locally, after the change I also have the exact same font appearance
as with native applications. Before the patch the fonts did use my
anti-aliasing settings iiuc. Before the patch, running the AppImage resulted
in lots of warnings:

```
Fontconfig warning: line 5: unknown element "its:rules"
Fontconfig warning: line 6: unknown element "its:translateRule"
Fontconfig warning: line 9: unknown element "description"
Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 4: unknown element "its:rules"
Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 5: unknown element "its:translateRule"
Fontconfig warning: "/etc/fonts/conf.d/10-hinting-slight.conf", line 8: unknown element "description"
...
```

... these warnings are now gone.

Thanks for the analysis by Friedrich.

Actually easy to test using the new AppImage format. Just do:

```
./KDevelop-5.3.0-x86_64.AppImage --appimage-extract
cd squashfs-root
./AppRun # to check appearance before
rm rm usr/lib/libfontconfig.so.1
./AppRun # to check appearance after rm'ing the lib
```

Reviewers: brauch, kossebau, cullmann

Reviewed By: cullmann

Subscribers: cullmann, kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D16893