build: remove LIBPTHREAD/LIBTHR build options

WITHOUT_LIBTHR has been broken for a little over five years now, since the
xz 5.2.0 update introduced a hard liblzma dependency on libthr, and building
a useful system without threading support is becoming increasingly more
difficult.

Additionally, in the five plus years that it's been broken more reverse
dependencies have cropped up in libzstd, libsqlite3, and libcrypto (among
others) that make it more and more difficult to reconcile the effort needed
to fix these options.

Remove the broken options.

PR: 252760

(cherry picked from commit 123ae3045dd21badb93ce52445e18e364b3ac807)
(cherry picked from commit 251a6ddfbdcd72e0de922e8320d2f0cc6806a423)