Delete deprecated VideoDecoder::InitDecode

Bug: webrtc:13045
Change-Id: Id1ca822c3be5a4f496dd67b59eab31a79a74bf67
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/228949
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34884}

api/test/mock_video_decoder.h

@@ -44,10 +44,6 @@ class MockVideoDecoder : public VideoDecoder {
   }
 
   MOCK_METHOD(bool, Configure, (const Settings& settings), (override));
-  MOCK_METHOD(int32_t,
-              InitDecode,
-              (const VideoCodec* codec_settings, int32_t number_of_cores),
-              (override));
   MOCK_METHOD(int32_t,
               Decode,
               (const EncodedImage& input_image,