do not show "LibreOffice Math" in the desktop menu (fdo#41681)

It is just a helper application. The desktop file still makes sense to
keep the MIME type association.

sysui/desktop/menus/math.desktop

 [Desktop Entry]
 Version=1.0
 Terminal=false
+NoDisplay=true
 Icon=math
 Type=Application
 Categories=Office;X-Red-Hat-Base;X-SuSE-Core-Office;X-MandrivaLinux-Office-Other;