removed some less workarounds for IE<11

[StefanSpieker]

Just two minor improvements:

• I guess the duplicate display definition was introduced to support IE < 11, because we do not need this anymore, according to caniuse display:inline-flex
• Similar case is with the ms- prefixed properties, the "normal ones" can all be used by IE 11, as stated:
  caniuse animation name
  caniuse animation duration
  caniuse animation iteration count
  caniuse animation timing function

Proposed changelog entries

• Minor: Removed some workarounds for IE < 11

Proposed upgrade guidelines

N/A

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[timja]

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!