Remove SaveTexture declaration in VideoCommon since it's only actually defined in OpenGL

Source/Core/VideoCommon/Src/ImageWrite.h

@@ -21,7 +21,6 @@
 #include "Common.h"
 
 bool SaveTGA(const char* filename, int width, int height, void* pdata);
-bool SaveTexture(const char* filename, u32 textarget, u32 tex, int width, int height);
 bool SaveData(const char* filename, const char* pdata);
 
 #endif  // _IMAGEWRITE_H