Remove unnecessary warnings from Designer Actions

Steps to Reproduce:

• In Designer, click on the UI menu, then Actions.
• Hover on the Create button, then click on the Operation button.
• Fill in the form according to this tutorial (but do not save yet).
• "Accidentally" select the sync-indexing checkbox (but do not save yet).
• Realize you've made a mistake and deselect the sync-indexing checkbox.
• Save.
  => A warning appears at the top of the form:

  Warnings
  "sync-indexing" is not an attribute of "nuxeo-operation-button".

  => "1 WARNING(S)" also appears next to the Action from the list of Actions.

Expected behavior:

• If sync-indexing is not an attribute of "nuxeo-operation-button", it shouldn't be available in the form. Otherwise, message is a false positive and should not appear.
• Warnings should be Warning(s).