Import/Use a preset - existing fields are not always deleted

I noticed this while developing on MDL-75492.

Test 1:

• Create a new database activity
• Go to presets and use Image gallery
• Go to fields and add a new field called "addfield"
• Go back to preset and use image gallery again
• In the mapping page do not check "Overwrite settings"
• Check that the new field "addfield" has disappeared

Test 1:

• Create a new database activity
• Go to presets and use Image gallery
• Rename all fields xxx to xxx2 (for example image to image2)
• Go back to preset and use image gallery again
• In the mapping page do not check "Overwrite settings"
• Check that the old field and new fields coexist (which should not be the case)

A solution has been found in MDL-75492 and can be backported.