devel/libpthread-stubs: remove port

libpthread-stubs >= 0.4 isn't granular: checks too few or often unused
pthread symbols. On FreeBSD it always added -pthread even if pthread
stubs libc would be enough. Consumers usually avoid libpthread-stubs
dependency on Linux, duplicating the logic. So, remove the wrapper.

PR: 269453
See also: https://gitlab.freedesktop.org/xorg/lib/pthread-stubs/-/issues/2
Inspired by: Arch, Fedora, OpenBSD, PkgSrc, Void
Approved by: ashish (maintainer)