Remove the Qt-specific ROI plotting

[astrofrog]

Thanks to improvements in Matplotlib, we can just define efficient selection tools directly in Matplotlib, which then means being able to use them in e.g. glue-jupyter with the Matplotlib viewers there.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e5dd5b1). Click here to learn what that means.
The diff coverage is 95.29%.

@@            Coverage Diff            @@
##             master    #1983   +/-   ##
=========================================
  Coverage          ?   87.76%           
=========================================
  Files             ?      243           
  Lines             ?    21662           
  Branches          ?        0           
=========================================
  Hits              ?    19012           
  Misses            ?     2650           
  Partials          ?        0

Impacted Files	Coverage Δ
glue/viewers/common/qt/toolbar_mode.py	89.84% <100%> (ø)
glue/viewers/image/qt/mouse_mode.py	45.58% <50%> (ø)
glue/core/roi.py	93.77% <96.05%> (ø)

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