Editorial: Remove "designed to be subclassable"

[ptomato]

See tc39/ecma262#2360

Bring the language around this in line with the main ECMA-262 text, using
the same unordered list form and the same bullet points everywhere.

[codecov]

Codecov Report

Merging #1677 (dba4162) into main (3337074) will decrease coverage by 22.22%.
The diff coverage is n/a.

❗ Current head dba4162 differs from pull request most recent head 5a7f958. Consider uploading reports for the commit 5a7f958 to get more accurate results

@@             Coverage Diff             @@
##             main    #1677       +/-   ##
===========================================
- Coverage   95.02%   72.79%   -22.23%     
===========================================
  Files          19       18        -1     
  Lines       10778     4838     -5940     
  Branches     1725     1058      -667     
===========================================
- Hits        10242     3522     -6720     
- Misses        523     1030      +507     
- Partials       13      286      +273     

Flag	Coverage Δ
test262	72.79% <ø> (ø)
tests	?

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Impacted Files	Coverage Δ
polyfill/lib/calendar.mjs	23.22% <0.00%> (-71.15%)	⬇️
polyfill/lib/intl.mjs	65.55% <0.00%> (-34.45%)	⬇️
polyfill/lib/instant.mjs	79.13% <0.00%> (-15.31%)	⬇️
polyfill/lib/plainmonthday.mjs	79.01% <0.00%> (-14.14%)	⬇️
polyfill/lib/plainyearmonth.mjs	83.42% <0.00%> (-12.99%)	⬇️
polyfill/lib/ecmascript.mjs	82.79% <0.00%> (-12.28%)	⬇️
polyfill/lib/plaindatetime.mjs	85.14% <0.00%> (-12.17%)	⬇️
polyfill/lib/timezone.mjs	81.66% <0.00%> (-11.80%)	⬇️
polyfill/lib/zoneddatetime.mjs	89.41% <0.00%> (-8.99%)	⬇️
polyfill/lib/duration.mjs	90.38% <0.00%> (-7.70%)	⬇️
... and 7 more

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