Revert of Move video_encoder.h and video_decoder.h to /api and create GN targets for them

webrtc/modules/video_coding/BUILD.gn

Index: webrtc/modules/video_coding/BUILD.gn
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index 9b611bb72f6ded94a411467cf3066225df407321..c6cbfca009d7400f9a7b4d8988535e1038fb08e4 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -129,7 +129,6 @@
 
   deps = [
     "../..:webrtc_common",
-    "../../api/video_codecs:video_codecs_api",
     "../../base:rtc_base_approved",
     "../../base:rtc_numerics",
     "../../base:rtc_task_queue",
@@ -225,7 +224,6 @@
   deps = [
     ":video_coding_utility",
     "../..:webrtc_common",
-    "../../api/video_codecs:video_codecs_api",
     "../../base:rtc_base_approved",
     "../../common_video",
     "../../system_wrappers",
@@ -324,7 +322,6 @@
       ":video_coding_utility",
       ":webrtc_vp8",
       "../..:webrtc_common",
-      "../../api/video_codecs:video_codecs_api",
       "../../base:rtc_base_approved",
       "../../common_video:common_video",
       "../../system_wrappers:system_wrappers",
@@ -531,7 +528,6 @@
       ":webrtc_vp9",
       "../..:webrtc_common",
       "../../api:video_frame_api",
-      "../../api/video_codecs:video_codecs_api",
       "../../base:rtc_base",
       "../../base:rtc_base_approved",
       "../../base:rtc_task_queue",