Replace repeated `qubit_maps` list with a single `qubit_map` in `execute_circuits`

[RoyStegeman]

Deferred from #1490 (comment). In the RB circuit execution, qubit_maps, is constructed as a list repeating the same qubit_map for every circuit:

qubit_maps = [qubit_map] * len(indexed_circuits)

This is a leftover from when a list of targets would generate one circuit per target and execute the circuits in a loop. Since #1490, all targets are executed in parallel on a single circuit, so qubit_map can be used directly in favor of qubit_maps.