Remove duplicate statically allocated variable

dsl_dataset_snapshot_sync_impl() declares `static zil_header_t zero_zil
__maybe_unused;`, but this is also declared globally. This wastes
memory.

CodeQL's cpp/local-variable-hides-global-variable check caught this.

Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14263