lib/posix-process: remove clone3 syscall

lib/posix-process: remove clone3 syscall

As clone3 currently does not work, and glibc falls
back to clone if clone3 is not available (tested on 2.35),
the syscall can be removed until a working implementation is
available.

Signed-off-by: Ioan-Teodor Teugea <teodor.teugea@gmail.com>
Reviewed-by: Florin Postolache <florin.postolache.of@gmail.com>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #847