Remove traits_super

[kitchoi]

Closes #1013

This PR removes traits_super.

Checklist

• Tests: Never tested.
• Update API reference (docs/source/traits_api_reference)
• Update User manual (docs/source/traits_user_manual) : Never mentioned
• Update type annotation hints in traits-stubs

[kitchoi]

Unrelated note: Thanks to the checklist which reminded me of the docs and annotations. I had initially forgotten!

[mdickinson]

Do we want to lose another blank line here?

(I had to find something to complain about. :-)

[kitchoi]

Yes! (Who knows if I deliberately left it there for it to be complained about ;) )

[mdickinson]

LGTM, CI permitting

[mdickinson]

Still LGTM!

[codecov-io]

Codecov Report

Merging #1015 into master will increase coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1015      +/-   ##
==========================================
+ Coverage   74.61%   74.81%   +0.20%     
==========================================
  Files          51       51              
  Lines        6457     6449       -8     
  Branches     1279     1279              
==========================================
+ Hits         4818     4825       +7     
+ Misses       1271     1253      -18     
- Partials      368      371       +3     

Impacted Files	Coverage Δ
traits/api.py	90.32% <ø> (ø)
traits/has_traits.py	72.40% <ø> (+0.23%)	⬆️
traits/editor_factories.py	79.59% <0.00%> (-9.19%)	⬇️
traits/trait_type.py	77.21% <0.00%> (-0.64%)	⬇️
traits/trait_types.py	72.31% <0.00%> (-0.15%)	⬇️
traits/trait_handlers.py	61.11% <0.00%> (+0.42%)	⬆️
traits/traits.py	76.63% <0.00%> (+4.09%)	⬆️
traits/etsconfig/etsconfig.py	63.46% <0.00%> (+6.41%)	⬆️

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