Remove tensorboard==1.14.0 version pinning from requirements.txt (#3185)
Remove tensorboard==1.14.0 version pinning from requirements.txt (#3185)

* requirements.txt: Remove pinned tensorboard version

PR #3041 tried two ways to control the version of tensorboard installed. Only one of them is needed, and the second, this one, causes a distracting warning: https://github.com/neuropoly/spinalcordtoolbox/issues/3145
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