drm/amd/display: remove unused variable oldest_index

cpp_check reports
drivers/gpu/drm/amd/display/modules/freesync/freesync.c:1143:17: style: Variable
  'oldest_index' is assigned a value that is never used. [unreadVariable]
   oldest_index = 0;
                ^

This variable is not used so remove.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>