target: Remove unneeded param pr_aptpl_buf_len to write_aptpl_to_file()

As mentioned in the comments in update_and_write_aptpl,
write_aptpl_to_file() calls strlen() on the buffer, and the length was
always being passed as zero.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>