Remove react-modal afterClose bug workaround

[gdostie]

Proposed Changes

Now that the afterClose prop bug has been closed (see reactjs/react-modal#755) we can safely remove the workaround we made in the ModalComposite component.

Potential Breaking Changes

Hopefully none.

Acceptance Criteria

• The proposed changes are covered by unit tests
• The potential breaking changes are clearly identified
• README.md is adjusted to reflect the proposed changes (if relevant)

[coveobot]

Live demo available here.

[codecov-io]

Codecov Report

Merging #1234 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1234      +/-   ##
==========================================
- Coverage   99.66%   99.66%   -0.01%     
==========================================
  Files         354      354              
  Lines        8926     8917       -9     
  Branches     2112     2111       -1     
==========================================
- Hits         8896     8887       -9     
  Misses         30       30

Impacted Files	Coverage Δ
...eact-vapor/src/components/modal/ModalComposite.tsx	100% <100%> (ø)	⬆️

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