clearOptions() deselects currently-selected items, but does not remove them as options

[dvg-p4]

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[replace me with a short description of issue]

Steps to reproduce:

1. Go to https://selectize.dev/docs/demos/api
2. Make a selection, e.g. "Star Chart"
3. Click the clearOptions() button

Expected result:

EITHER:

• The behavior described by the current API docs:
  • The current selection is cleared
  • All options are removed
• (This was the behavior of selectize.js < v0.12.5)

OR:

• The behavior described by the changelog:
  • Any non-selected options are removed
  • The current selection remains selected
• This was requested by clearOptions should not clear items #593 and How to dynamically update options on type #1079 and implemented by f6c4f7c; it was the behavior of selectize.js ≥ v0.12.5, < v0.13.1

Actual result:

• Only non-selected items are removed from the options
• Any current selections are cleared, but remain as options

Cause:

#1553 (released in v0.13.1) added back the self.clear() to the end of the clearOptions code, but did not remove the logic change that prevents currently-selected items from being removed from the list of options. This PR was intended to close #832 , but that request only made sense before the behavior of clearOptions() was changed (in v0.12.5) to not affect the current selection. I suspect that this oversight was due to v0.12.5 not updating the docs to clarify that selected items were intended to be kept--the newer PR also updated the docs to imply that the pre-0.12.5 behavior was intended.