[Blink] Remove DrawingBuffer HW TransferableResource alpha_type override

The alpha type of the SI used to back the TransferableResource here is
set at [1][2]. `back_buffer_alpha_type` will be premul [3] unless
`requested_alpha_type_` is unpremul and SCANOUT isn't added as an SI
usage [4]. If `requested_alpha_type_` is unpremul and
`back_buffer_alpha_type` is set to premul, then
`premultiplying_internally_` will be set to true. Hence, the value of
`back_buffer_alpha_type` matches the value that will be computed for
the alpha type in the override passed to the TransferableResource:

* `requested_alpha_type_` is premul -> premul
* `requested_alpha_type_` is opaque -> premul
* `requested_alpha_type_` is unpremul -> premul if
  `premultiplying_internally_` is true, otherwise unpremul

Hence, we can remove the override.

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc;l=1968;drc=743a82d08e59d803c94ee1b8564b8b11dd7b462f?q=back_buffer_alpha&ss=chromium
[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc;l=2021;drc=743a82d08e59d803c94ee1b8564b8b11dd7b462f?q=back_buffer_alpha&ss=chromium
[3] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc;l=1962;bpv=1;bpt=1?q=back_buffer_alpha&ss=chromium
[4] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/drawing_buffer.cc;l=2014-2017;drc=743a82d08e59d803c94ee1b8564b8b11dd7b462f?q=back_buffer_alpha&ss=chromium

Bug: 410591523
Change-Id: Id79f4bba70b1577b0a94a6783024cf3e3b3eb773
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6573572
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1463935}