xxhash: remove xxh3 (does not compile)

we do not use that anyway.

src/borg/algorithms/xxh64/xxhash.h

@@ -1920,16 +1920,6 @@ XXH_PUBLIC_API XXH64_hash_t XXH64_hashFromCanonical(const XXH64_canonical_t* src
     return XXH_readBE64(src);
 }
 
-
-
-/* *********************************************************************
-*  XXH3
-*  New generation hash designed for speed on small keys and vectorization
-************************************************************************ */
-
-#include "xxh3.h"
-
-
 #endif  /* XXH_NO_LONG_LONG */