Remove functional background from background settings

[backmari]

Short description of the changes:

Remove functional background from background settings. The backend lr_reduction will make the decision about functional background depending on whether the option to use two backgrounds is active or not.

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

Open the background settings in the UI and check that the functional background is removed and the remaining options work as before.

References

Defect 4326: [REFRED] Eliminate "functional background" from the background settings

[jmborr]

👌