Revert "Reduce stack usage in tls13_hkdf_expand"

This reverts commit ec0c5f5693e39c5a013f81e6dd9dfd09ec65162d.

SSL_export_keying_material() may use longer label lengths.

Fixes #7712

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7755)

(cherry picked from commit ed371b8cbac0d0349667558c061c1ae380cf75eb)