* gnu/packages/qt.scm (qtwebengine-5)[source](snippet): Move code to delete
third-party files out to ...
(remove-third-party-files): ... this new variable.
(qtwebengine)[source](snippet): Use that variable instead of the broken
duplicate implementation. Adjust PRESERVED-THIRD-PARTY-FILES accordingly.
[native-inputs]: Add PYTHON-BEAUTIFULSOUP4.