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