Remove unnecessary `polyfill.io` (#3763) #495

Job	Run time
Build demo site with base requirements (3.11)	36s
Linting (flake8, pydocstyle) (3.12)	17s
Build demo site with base requirements (3.12)	24s
Baseline testing (Python 3.12 on ubuntu-latest)	44s
Nikola tests (Python 3.8 on ubuntu-latest)	1m 56s
Nikola tests (Python 3.9 on ubuntu-latest)	1m 54s
Nikola tests (Python 3.10 on ubuntu-latest)	2m 25s
Nikola tests (Python 3.11 on ubuntu-latest)	2m 5s
Nikola tests (Python 3.12 on ubuntu-latest)	2m 7s
Nikola tests (Python 3.12 on macos-latest)	3m 28s
Nikola tests (Python 3.12 on windows-latest)	3m 55s
	19m 51s

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