Remove is_remapped flag from rr-edges

[vaughnbetz]

This flag is on every rr-edge, which is wasteful. It is really providing some high level control functionality only during rr-graph building.

Best solution if possible: have the control code decide whether or not to remap some/all switches, with no storage.

If not possible, put an unordered_set in the rr_graph_builder object so we only temporarily have this storage. Set the flag only if we need to remap something. Could alternatively put it in a bool vector [0..num_rr_edges-1].

[amin1377]

This issue is addressed in this pull request. The flag cannot be removed because, when flat routing is enabled, remapping on edges is performed twice. Therefore, we require a way to identify the edges that have already been remapped. Otherwise, we may encounter a situation where an edge is remapped twice. However, this variable is only needed during construction of the RR (Routing Resource) graph. Consequently, I have deleted the vector that stores this variable once the RR graph is built.