API dox: use UTF-8 infinity character instead of generating png via TeX
My openSUSE TW TeX seems to fail to process this, at least by whatever means it is called by doxygen.
api.kde.org server also currently misses tex installtion.
As we seem to have standardized on UTF-8 as format for KDE source code files, using the UFT-8 character directly seems a better way these years. Doxygen at least in 1.8.16 has no issues dealing with it.
Cmp. https://api.kde.org/kdepim/kimap/html/classKIMAP_1_1ImapSet.html just showing the broken result currently.
Edited by Friedrich W. H. Kossebau