fix(deploy): clean up gh-pages obsolete files (#3081)

[rolyatsats]

Prevents the gh-pages branch from growing indefinitely by cleaning up before copying new files.
Fixes #3081

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[rolyatsats]

This process runs git ls-files, removes .gitignore and 404.html from the list, then passes this to git rm -r by wrapping each file path in double quotes such as:

git rm -r "style.css" "index.html" "main.js"

This works because the files are then added back when copying from dist before finally being committed.

[googlebot]

CLAs look good, thanks!

[hansl]

LGTM. Thanks!

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