various: do not use assert_se as a workaround in non-test code

various: do not use assert_se as a workaround in non-test code

This partially reverts 5332be60d3897c7b86d28cf7b9d61c5dc6847fd6. I expect that
there is no practical difference, but it seems philosophically wrong to use
assert_se(), i.e. for the generation of the code in non-debug builds, just to
suppress a warning. We have _unused_ for that, use it.

I verified that we don't get warnings with clang and -DNDEBUG=1 with this patch.