ui789 redirect links fixes

UI ticket: https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/-/issues/789

While the ticket got closed and split into two parts, this MR is still relevant, as it fixes the documentation link specs to reflect how the redirects are currently behaving.

Changes:

• removes duplicated help_center_abp_en
• adds missing /$lang/ to target links with locale support for lang parameter based redirects
• removes /$lang/ from target links with locale support for redirects based on the language in the HTTP header
• fixes wrong link for adblock_plus_report_issue, lt locale
• removes a parameter separator (?) at end of target urls
• removes entries redirecting to 404 pages
• removes IF APL from eyeo and filterdoc