WPrefs: remove unused constants ICON_TITLE_*FONT
Apparently, there was a time when it was though to add a label below (?)
the icons of the list of panels, but this has never been finished and the
unfinished code to do this was even removed in commit
0e01e6920572b3d3a7a8df2872513144225440a8
So, as it is unlikely that the "feature" will return, and if it did the
font should be handled in a better way, this patch removes the definition
of the constants with the names for the font used for those labels.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>