Remove unused AddDataBuffer declaration

The AddDataBuffer method was declared but never defined
in any implementation of CSoundData or its derived classes.

It is also not used anywhere in the codebase, so this
commit removes the dead declaration to clean up the
interface.

source/soundmanager/data/OggData.h

@@ -54,7 +54,6 @@ protected:
 	ALuint m_Buffer[100];
 	int m_BuffersUsed;
 
-	bool AddDataBuffer(char* data, long length);
 	void SetFormatAndFreq(int form, ALsizei freq);
 	int  GetBufferCount();
 	unsigned int GetBuffer();