[4.x] Remove clear button of readonly date field

[PatrickJunod]

Change:

Removed the "x" button from the date field / date range field when it is readonly.

Reasoning:

Removing this button eliminates the option to empty the date field. This resolves the issue where, if the date field is required but left empty, the page cannot be saved.

Example:

Before, we could click on the cross to clear this field, thus making it impossible to save the entry if the date field is required :

Before this fix:

After (removed the x button):