removed dependency on option list of custom option

[anishagg17]

Summary

Fixes: #2258

Replaced return flattenOptionGroups(options).length === selectedOptions.length; this equality check with more robust and accurate check within the EuiComboBox component which checks for length equality without including any of the custom option

Checklist

- [ ] Check against all themes for compatibility in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in IE11 and Firefox
- [ ] Props have proper autodocs
- [ ] Added documentation examples
- [ ] Added or updated jest tests

• Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[anishagg17]

@snide should the entry in Changelog.md be under breaking-changes section

[chandlerprall]

The removal of onCreateOption's second parameter, options, doesn't seem to be necessary? Would be nice to avoid making this a breaking change.

[anishagg17]

Changes have been made

[chandlerprall]

The onCreateOption definition & docs examples shouldn't need to be changed for this, either. I think only that logic change in combo_box.tsx is necessary.

[anishagg17]

Updated accordingly

[anishagg17]

I optimized it a bit more by mapping label of each flattenOptionGroups(options) to option.label.trim().toLowerCase()

[chandlerprall]

jenkins test this

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3183/

[chandlerprall]

Changes LGTM! Pulled & tested locally