Revert of Move video_encoder.h and video_decoder.h to /api and create GN targets for them (patchset #8 id:140001 of https://codereview.webrtc.org/2780943003/ )

Reason for revert:
Suspect of breaking Chrome FYI bots.

See
https://build.chromium.org/p/chromium.webrtc.fyi/builders/Mac%20Builder/builds/23065
https://build.chromium.org/p/chromium.webrtc.fyi/builders/Android%20Builder

Example logs:
../../content/renderer/media/gpu/rtc_video_encoder_unittest.cc:18:46: fatal error: third_party/webrtc/video_encoder.h: No such file or directory
 #include "third_party/webrtc/video_encoder.h"
                                              ^

Original issue's description:
> Move video_encoder.h and video_decoder.h to /api and create GN targets for them
>
> BUG=webrtc:5881
> # Because PRESUBMIT ignores LINT blacklist for moved files and these
> # headers have some not easy to resolve issues.
> NOPRESUBMIT=True
>
> Review-Url: https://codereview.webrtc.org/2780943003
> Cr-Commit-Position: refs/heads/master@{#17511}
> Committed: c42f540570

TBR=solenberg@webrtc.org,sprang@webrtc.org,ilnik@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5881

Review-Url: https://codereview.webrtc.org/2794033002
Cr-Commit-Position: refs/heads/master@{#17514}

webrtc/modules/video_coding/include/video_codec_interface.h

@@ -14,12 +14,12 @@
 #include <vector>
 
 #include "webrtc/api/video/video_frame.h"
-#include "webrtc/api/video_codecs/video_decoder.h"
-#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/modules/video_coding/include/video_error_codes.h"
 #include "webrtc/typedefs.h"
+#include "webrtc/video_decoder.h"
+#include "webrtc/video_encoder.h"
 
 namespace webrtc {