Remove the heptapod.mirror-path setting

This HGRC setting tells our inner Git synchronization hook where its target is.

But there is currently only one working value, it'd be trivial from the hook to know it. Therefore, this setting is a useless liability, keeping it to the appropriate value is useless work (e.g, we'll want to switch protocols after #73 (closed)) and leads to various bugs like #90 (closed), when we forget to do it. It's also a source of trouble if users start to customize their HGRC, or if we want to make the HGRC editable from the Web UI.

Hence we should remove it, or at least give it a default, computed value.

Removing it with the appropriate migration step would automatically fix #90 (closed) for all affected projects.