Revert "Reduce stack usage in tls13_hkdf_expand"

This reverts commit ec0c5f56

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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 ed371b8c)