Remove unused header from the Zstandard compression backend.

Since we stopped looking at Zstandard error codes there is no need to
include the zstd_errors.h header file anymore.

src/common/compress_zstd.c

@@ -20,7 +20,6 @@
 
 #ifdef HAVE_ZSTD
 #include <zstd.h>
-#include <zstd_errors.h>
 #endif
 
 /** Total number of bytes allocated for Zstandard state. */