Remove pandas version bound prior to release

[wshanks]

#1364 pinned the max version of pandas to avoid CI failures, but we should try to make qiskit-experiments compatible with the latest stable version of pandas prior to release.

[coruscating]

@ItamarGoldman This should be resolved by #1360 so you don't need to work on this.

[wshanks]

Closed by #1319