media: Delete renderer/demuxer splicing code.

As of https://codereview.chromium.org/2343543002/, crossfaded splicing
is no longer a thing. Now we simply trim overlapped buffers.

This CL deletes the remaining splicing code.

BUG=588351, 577438

Review-Url: https://codereview.chromium.org/2492953003
Cr-Commit-Position: refs/heads/master@{#431488}
20 files changed