win32: remove workaround for font backend
It turns out that explicitely setting font backend on win32 is only useful for pygobjectwin32 PangoCairo builds because in vanilla PangoCairo win32 backend is default on windows and changing default to fontconfig was patch included in pygobjectwin32 since
- PangoCairo: favor fontconfig over win32 fontmap by default, this would overcome "Pango Warning:... expect ugly output"
Recent libraries update in msys2-mingw64 breaks current code (according to https://github.com/Alexpux/MINGW-packages/pull/3611 the breakage reason is PangoCairo.Win32FontMap being private)
While meld code can be modified to use non-private API it looks unneeded since meld requires gtk 3.20 and pygobjectwin32 builds stalled with gtk 3.18 and can't run current meld 3.19+
So font backend setting is removed. Also package presence check is removed since there in no other explicit uses in meld.