Remove SegmentedBuffer::MergeSegmentsIntoBuffer() method

This method is not used after https://crrev.com/c/5595081

Bug: 342859647
Change-Id: Ic6af45426a31161608b33b6b8f88dc8e4a14b86c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5612372


Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312676}