Drop CSS hack for uBlock Origin sidebar

Commit 63d11a4da6e0d2400c104a17859687fff9396b40 on devel and the corresponding commit on stable (should be there once #16728 (closed) is merged) should be reverted, because according to the commit message: “once we ship Tor Browser 9.0 based on Firefox 68, we can remove this hack”.

Related issues

Related to #16206 (closed)
Related to #16858 (closed)
Related to #16728 (closed)
Blocks #16209

Original created by @segfault on 16973 (Redmine)

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