lz4_decompress_abd declared but not defined

`lz4_decompress_abd` is declared in zio_compress.h but it is not defined
anywhere. The declaration should be removed.

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
External-issue: DLPX-47477
Closes #8894

include/sys/zio_compress.h

@@ -105,8 +105,7 @@ extern size_t lz4_compress_zfs(void *src, void *dst, size_t s_len, size_t d_len,
     int level);
 extern int lz4_decompress_zfs(void *src, void *dst, size_t s_len, size_t d_len,
     int level);
-extern int lz4_decompress_abd(abd_t *src, void *dst, size_t s_len, size_t d_len,
-    int level);
+
 /*
  * Compress and decompress data if necessary.
  */