Removing VCMCodecDataBase::Codec and VideoCodingModule::Codec.

This CL brings us one step closer to removing CodecDatabase and
GenericEncoder, by removing the static VCM::Codec(). Codec specific
methods are moved to video_encoder.cc (they already belonged to this
class) and getting default generic codec settings has been moved to a
test specific file.

This CL also makes video_encoder.h pass style guide and lint checks,
since these checks are triggered with the new video_encoder.cc file.

BUG=webrtc:8064

Review-Url: https://codereview.webrtc.org/2993923002
Cr-Commit-Position: refs/heads/master@{#19303}

webrtc/modules/video_coding/include/video_coding_defines.h

@@ -40,7 +40,6 @@ namespace webrtc {
 #define VCM_NOT_IMPLEMENTED -20
 
 enum {
-  kDefaultStartBitrateKbps = 300,
   // Timing frames settings. Timing frames are sent every
   // |kDefaultTimingFramesDelayMs|, or if the frame is at least
   // |kDefaultOutliserFrameSizePercent| in size of average frame.