Remove redundant declaration for zfs_crc64_table

zfssubr.c already defines this statically. Besides, zfsimpl.c defined it, but
didn't use it.

This fixes a -Wredundant-decls warning.

MFC after: 3 days
Reported by: amd64-gcc-6.3.0 (devel/amd64-xtoolchain-gcc)
Sponsored by: Dell EMC Isilon