testing: drop unneeded vga/vnc stanzas from QEMU invocation

The aarch64 virt platform doesn't have any default VGA devices so we
don't need to configure them here.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Julien Olivain <ju.o@free.fr>

support/testing/tests/package/test_flutter.py

@@ -48,8 +48,6 @@ class TestFlutter(infra.basetest.BRTest, GraphicsBase):
                      "-cpu", "cortex-a57",
                      "-m", "512M",
                      "-smp", "4",
-                     "-vga", "std",
-                     "-vnc", "none",
                      "-drive", f"file={img},if=virtio,format=raw"])
         self.emulator.login()