Retire the minix3 test config.

It got broken by the sshd-auth change, it's not obvious why, and the
platform lacks the debugging tools (eg gdb, strace) to figure it out.
The upstream project seems effectively dead (6 years since the last
commit, 10 since the last release).  It was useful while it lasted
(we found a real bug because of it) but its time seems to have passed.

.github/workflows/selfhosted.yml

@@ -40,7 +40,6 @@ jobs:
           - fbsd12
           - fbsd13
           - fbsd14
-          - minix3
           - nbsd3
           - nbsd4
           - nbsd8