build-sys: drop needless warning pragmas for old pixman

Since commit 236f282c ("configure: check for pixman-1 version"), QEMU
requires >= 0.21.8.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>