A lot of unused variables

I assigned the task myself because I plan to fix this issue and make a Merge Request.
Please do not work on this issue.

I did not delete those unused variables that have TODO comments next to them, which means that these variables can be used in the future.

Please review MR. Thanks!

Took the opportunity and also removed unused namespaces too.

Please ensure that this is not being reverted: https://www.drupal.org/project/field_tools/issues/3324488 🐛 Field groups are not copied when cloning display RTBC

Looks like this needs an update!

MR now requires conflict resolution anyway so I set it as "Needs work".
As for your last comment @anybody, maybe we should check this ticket after #3324488 is resolved?

MR fixed. Still have to decide what to do related to 🐛 Field groups are not copied when cloning display RTBC .