drm/amd/display: Remove unused dmub_outbox_irq_info_funcs variable

Remove the unused struct irq_source_info_funcs
dmub_outbox_irq_info_funcs from the file, which was declared but never
hooked up.

This was pointed by clang with the following warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/irq/dcn201/irq_service_dcn201.c:141:43:
warning: unused variable 'dmub_outbox_irq_info_funcs'
[-Wunused-const-variable]
static const struct irq_source_info_funcs dmub_outbox_irq_info_funcs = {
                                          ^
Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/irq/dcn201/irq_service_dcn201.c

@@ -138,11 +138,6 @@ static const struct irq_source_info_funcs vupdate_no_lock_irq_info_funcs = {
 	.ack = NULL
 };
 
-static const struct irq_source_info_funcs dmub_outbox_irq_info_funcs = {
-	.set = NULL,
-	.ack = NULL
-};
-
 #undef BASE_INNER
 #define BASE_INNER(seg) DMU_BASE__INST0_SEG ## seg