kern: pts: do not special case closed slave side

This would previously return 1 if the slave side of the pts was closed
to force an application to read() from it and observe the EOF, but it's
not clear why and this is inconsistent both with how we handle devices
with similar mechanics (like pipes) and also with other kernels, such as
OpenBSD/NetBSD and Linux.

PR: 239604
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D43457