fix(list): Remove unused adapter.removeAttributeForElementIndex

[moog16]

While implementing react list, I noticed this method is actually not used in the foundation.

[codecov-io]

Codecov Report

Merging #4473 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4473      +/-   ##
==========================================
- Coverage   99.05%   99.05%   -0.01%     
==========================================
  Files         130      130              
  Lines        6235     6230       -5     
  Branches      811      810       -1     
==========================================
- Hits         6176     6171       -5     
  Misses         58       58              
  Partials        1        1

Impacted Files	Coverage Δ
packages/mdc-list/component.ts	98.46% <ø> (-0.05%)	⬇️
packages/mdc-list/foundation.ts	100% <ø> (ø)	⬆️

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[kfranqueiro]

I think this seems reasonable, but it occurs to me we're going to need to start noting even adapter API removals as BREAKING CHANGE because it will break anyone using the adapter type in TypeScript if they're implementing a superset.

cc @abhiomkar if he wants to take a look at this too.

[moog16]

Ya - I also thought this is a breaking change even if its not being used. Users should at least be aware via the CHANGELOG.

[mdc-web-bot]

All 627 screenshot tests passed for commit 69a9858 vs. master! 💯🎉

[mdc-web-bot]

All 627 screenshot tests passed for commit 78d8587 vs. master! 💯🎉