Functionality of hide local tasks is missing in 2.x

Ready incl. phpunit tests.

Please review and feedback.

I noticed that the new feature to hide the local tasks works perfectly fine when using the default 'Olivero' theme, but it didn't have any impact when using a custom theme. So in case the theme removes the class 'block-local-tasks-block' the default local tasks tab isnt't removed. IMO a note for the admin regarding this, along with the settings or in the README, would be helpful.

Added text to admin form and README. Please have a look and give feedback or fix. Thank you in advance.

Looks good!! Moving to RTBC.

Thank you all for your work. This issue is merged.