Remove deprecared avcodec_close()

From [docs](https://ffmpeg.org/doxygen/trunk/deprecated.html) see that
this function call is not needed:

"Do not use this function. Use avcodec_free_context() to destroy a codec
context (either open or closed). Opening and closing a codec context
multiple times is not supported anymore – use multiple codec contexts instead."

avcodec_free_context is already called.

include/usb_cam/formats/mjpeg.hpp

@@ -164,7 +164,6 @@ class MJPEG2RGB : public pixel_format_base
       free(m_avoptions);
     }
     if (m_avcodec_context) {
-      avcodec_close(m_avcodec_context);
       avcodec_free_context(&m_avcodec_context);
     }
     if (m_avframe_device) {