Vulkan: Replace BufferVk::getBufferAndOffset() with getBuffer() Now BufferHelper class already keeps offset information. There is no reason for BufferVk to have that information any more. Bug: b/205337962 Change-Id: I6e014fb480bfcd5018ef9231b0fb87a50021f179 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3266147 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com>