Drop advertisement clause on Blowfish
Originally driven by https://github.com/pyca/bcrypt/issues/169, OpenBSD removed Niels Provos's BSD advertisement clause in version 7.1:
https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/lib/libsa/blowfish.c.diff?r1=1.1&r2=1.2 https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/lib/libsa/blowfish.h.diff?r1=1.1&r2=1.2
Dropping this GPL-incompatible clause enables using libssh in GPL software. libssh2 has integrated the change already: https://github.com/libssh2/libssh2/commit/91adeae486105ae25823949eea4d36c7587043bd