tests: Enable sha256 testing

Some tests don't need any modification for being run with sha256 support. Let's remove the build flag block for such tests.

These tests were found using a simple fish shell script which ran tests by removing the build blocker and noted tests which didn't fail.

for i in (grep -Iirl "//go:build !gitaly_test_sha256" --include "*test.go")
    ex -c "1d" -c "1d" -c ":wq" $i
    if grep -Iirl "//go:build !gitaly_test_sha256" (dirname $i)/testhelper_test.go
        echo "teshelper cleanup"
        ex -c "1d" -c "1d" -c ":wq" (dirname $i)/testhelper_test.go
    end
    echo "=========> running $i"
    GITALY_TESTING_ENABLE_SHA256="YesPlease" TEST_PACKAGES=./(dirname $i) make test-go 1>/dev/null
    if test $status -eq 0
        echo "success: $i"
        echo $i >> good
    else
        echo "fail: $i"
    end
    git checkout $i
end