UPSTREAM: drm/mst: delete unnecessary case in drm_dp_add_payload_part2()

[Why]
There is no need to consider payload->delete case since we won't call
drm_dp_add_payload_part2() to create a payload when we're about to
remove it.

[How]
Delete unnecessary case to simplify the code.

Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230807025639.1612361-2-Wayne.Lin@amd.com


(cherry picked from commit ae4d2314)

BUG=b:332903004, b:322223603
TEST=Verify the link training fallback for UHBR<->non-UHBR rates

Change-Id: I567f7fe8afa57d3d6c738444825f075591d7c6da
Signed-off-by: Rakshith M O <rakshith.m.o@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5802894


Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Manasi Navare <navaremanasi@chromium.org>
Signed-off-by: Hubert Mazur <hmazur@google.com>