lib/core: (refactor) Drop wrapper and unneeded args for variant writing

lib/core: (refactor) Drop wrapper and unneeded args for variant writing

Nothing was using the `bytes_written` data (we always discard partially written
tmpfiles), so simplify everything by dropping it. Further, we always passed an
offset of `0`, so drop that argument too. (I believe that this was previously
used by the "pack files" code that we deleted long ago)

Second, we had an unnecessary internal wrapper for this function; drop that too.

Closes: #1257
Approved by: jlebon