media/gpu/test: Remove video_decode_accelerator_unittest documentation.

The video_decode_accelerator_unittest is deprecated in favor of the
video_decode_accelerator_tests and video_decode_accelerator_perf_tests.
This CL removes all documentation related to the old tests.

vdatest_usage.md documents both the VEA and VDA tests. It has been renamed to
veatest_usage.md and all parts referring to the VDA tests have been removed.

TEST=None

BUG=1802897

Change-Id: I754c039238e553493bc4076b160ab6928e97a4c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1800853
Commit-Queue: David Staessens <[email protected]>
Reviewed-by: Hirokazu Honda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#709791}
diff --git a/docs/README.md b/docs/README.md
index 2072fbe..bc8ebd0 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -303,12 +303,12 @@
 *   [Autoplay of HTMLMediaElements](media/autoplay.md) - How HTMLMediaElements
     are autoplayed.
 *   [Piranha Plant](piranha_plant.md) - Future architecture of MediaStreams
-*   [Video Decode/Encode Accelerator Tests](media/gpu/vdatest_usage.md) - How to
-    use the accelerated video decoder/encoder test programs.
+*   [Video Encode Accelerator Tests](media/gpu/veatest_usage.md) - How to
+    use the accelerated video encoder test program.
 *   [Video Decoder Tests](media/gpu/video_decoder_test_usage.md) - Running the
-    new video decoder tests.
+    video decoder tests.
 *   [Video Decoder Performance Tests](media/gpu/video_decoder_perf_test_usage.md) -
-    Running the new video decoder performance tests.
+    Running the video decoder performance tests.
 
 ### Accessibility
 *   [Accessibility Overview](accessibility/overview.md) - Overview of