Makefile: NO_OPENSSL=1 should no longer imply BLK_SHA1=1

Makefile: NO_OPENSSL=1 should no longer imply BLK_SHA1=1

Use the collision detecting SHA-1 implementation by default even when
NO_OPENSSL is set.

Setting NO_OPENSSL=UnfortunatelyYes has implied BLK_SHA1=1 ever since
the former was introduced in dd53c7ab29 (Support for NO_OPENSSL,
2005-07-29).  That implication should have been removed when the
default SHA-1 implementation changed from OpenSSL to DC_SHA1 in
e6b07da278 (Makefile: make DC_SHA1 the default, 2017-03-17).  Finish
what that commit started by removing the BLK_SHA1 fallback setting so
the default DC_SHA1 implementation will be used.

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>