src: remove `kFlagNoShutdown` flag

This was originally introduced in 3446ff41, in order to fix
a hard crash. However, since the libuv 1.18.0 update, that hard
crash is gone, and since f2b9805f we do not throw an
error in JS land anymore either, rendering the flag unnecessary.

Also, the original test that checked this condition was added
to `test/parallel/`. Since that typically runs without a TTY stdin,
a duplicate test is being added to the pseudo-tty test suite
in this commit.

Refs: https://github.com/nodejs/node/commit/3446ff417ba1e11d35d1661b8788eac5af029360
Refs: https://github.com/nodejs/node/commit/f2b9805f85d3ff770892b37944a0890e0e60ca78
Refs: https://github.com/libuv/libuv/commit/0e2814179c9903423d058b095e84f48fcfb8f3d1
PR-URL: https://github.com/nodejs/node/pull/20388


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>