chinese/phpbb-tw: fix hostname in pkg-message for reproducible builds

As with r417677 for sysutils/3dm, avoid %%HOSTNAME%% in the pkg-message.
The host on which the port/package is installed is relevant, not that on
which it was built.

Approved by: swills
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7234