box: use OpenSSL version of SHA1 in chap-sha1 implementation

We added an internal version of SHA1, because initially we didn't link
the OpenSSL library. However, since commit 59a55740 ("Link against
libssl and libcrypto. Issue #1382") we do so there's no need in it.

Part of #7987

NO_DOC=code cleanup
NO_TEST=code cleanup
NO_CHANGELOG=code cleanup