Cleanup : directly include of `internal/nelem.h` when required.

And so clean a few useless includes

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19721)

(cherry picked from commit f2a6f838)