Fixed #1804: URL field is not removed by integrity check

[oscargus]

Better to disable the check than remove the field from the entry. ;-)

Fixes #1804

• Change in CHANGELOG.md described
• Tests created for changes (one may think of adding a test that no fields are removed from the entry in the integrity check, but I haven't done that here, ideally the entry should have every possible field included...)
• Manually tested changed features in running JabRef
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[matthiasgeiger]

LGTM