Remove the usage of undefined QFontDatabase::WritingSystem

There is no definition for Yi, Tagalog...Braille in
QFontDatabase::WritingSystem, see the header file.
(cherry picked from commit qtbase b7b858595e6e2d8dcc9debeb45eb8b9b294b9be6)

Change-Id: I0d3625c58310d9af680799d20867f04b42845ca9
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>