libc: Remove readdir_r(3)

This function was never safe to use. We marked it deprecated in the
manual page in 2016, and it is marked obsolete in POSIX 2024. We
previously added a linker warning and annotated the prototype; now that
stable/15 has been branched, we can remove it from main.

Relnotes: yes
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D52474