drm/amd/display: Remove unused dmub_outbox_irq_info_funcs variable

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>