mesa/get: Remove unused extra_ARB_viewport_array

Unused since 0a7691ee (mesa: Enable enums for OES_viewport_array).
Silence a warning of unused variable.

Signed-off-by: Boyan Ding <boyan.j.ding@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/src/mesa/main/get.c b/src/mesa/main/get.c
index 10b40d4..f0bb041 100644
--- a/src/mesa/main/get.c
+++ b/src/mesa/main/get.c
@@ -482,7 +482,6 @@
 EXTRA_EXT(ARB_shader_atomic_counters);
 EXTRA_EXT(ARB_draw_indirect);
 EXTRA_EXT(ARB_shader_image_load_store);
-EXTRA_EXT(ARB_viewport_array);
 EXTRA_EXT(ARB_query_buffer_object);
 EXTRA_EXT2(ARB_transform_feedback3, ARB_gpu_shader5);
 EXTRA_EXT(INTEL_performance_query);