Previously, jQuery 1.12.4 was added to the WordPress SVN repo in order to backport an upstream security patch from jQuery 3.4.0.
Now that WordPress includes an unmodified version of jQuery 3.5.1 and no longer needs to maintain a fork, it can be removed from the SVN repo and installed via NPM again as part of the build.
Follow-up to [45342], [49101].
Props peterwilsoncc, SergeyBiryukov.
Fixes#52647.
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