build-sys: remove some gtk+ 2.0 warnings flags
build-sys: remove some gtk+ 2.0 warnings flags

As we dropped gtk+ 2.0 anyway.

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@gmail.com>

flexVDI/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk