mbstream: Remove duplicate definition of datasink_buffer

For some reason, when mbstream is compiled with GCC 10,
ld will fail to link it due to a multiple definition of
datasink_buffer. The code appears to have been like that
since the very beginning.

Let us remove the duplicate dummy definition.

extra/mariabackup/xbstream.c

@@ -45,7 +45,6 @@ datasink_t datasink_archive;
 datasink_t datasink_xbstream;
 datasink_t datasink_compress;
 datasink_t datasink_tmpfile;
-datasink_t datasink_buffer;
 
 static run_mode_t	opt_mode;
 static char *		opt_directory = NULL;