Docs: Drop redundant region variable from various equations

[alexdewar]

In a number of places in dispatch_optimisation.md we sum over regions, like so:

$$ min. \sum_{r}{\sum_{a}{\sum_{c}{\sum_{ts}}}} cost_{r,a,c,ts} * q_{r,a,c,ts} $$

In many cases though, the region variable is probably not relevant. In fact, in the code we don't currently group assets by region at all, as region doesn't play a role. This will change at some point (e.g. I think we will want commodity flows to be balanced within regions), but in any case I think there are a bunch of places where we can drop the rs and make things simpler and clearer.

See also #382.