gnu: ungoogled-chromium: Unbundle openjpeg again.

This is a follow-up to commit 40594fdcb9e360d68e5444771b5467ba5bbbfcf2.

* gnu/packages/chromium.scm (%preserved-third-party-files): Remove
'third_party/pdfium/third_party/libopenjpeg20'.
(ungoogled-chromium)[arguments]: Fix typo in #:configure-flags.