drm/amdkfd: remove unused kfd_pm4_headers_diq header file

kfd_pm4_headers_diq.h header is a leftover from the old H/W debugger
module support added on commit
fbeb661b ("drm/amdkfd: Add skeleton H/W debugger module support").
That implementation was removed after a while and the last file that
included that header was removed on commit
5bdd3eb2 ("drm/amdkfd: Remove unused old debugger implementation").

This patch removes the unused header file kfd_pm4_headers_diq.h

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>