Remove warnings about invalid background range

[backmari]

Short description of the changes:

The user is shown warnings when the peak range is not inside the background range. However, these warnings are obsolete as it is now valid to to define one or two background regions away from the peak range.

This PR removes the warnings shown here (red stars and text "INVALID SELECTION"):

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

Test changing peak and background ranges in the popup plots that appear when double-clicking either the Y vs TOF plot or the counts vs Y plot. Everything should work as before, except that there should be no red warnings about "INVALID SELECTION" shown.

References

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

[jmborr]

👌