How to delete all data in one field of a particular content type?
If you only need to do this once you could use Views Bulk Operations https://www.drupal.org/project/views_bulk_operations

/admin/structure/views


Click "Add new view" link
Show "Content" of type "YOUR_CONTENT_TYPE" 
Click "Continue &amp; Edit" button


Under the FORMAT section change Show: click 'Content' change to 'Fields' 


Click 'Apply' button
Click 'Apply' button again


Under FIELDS section click the 'Add' button


Checkmark 'Bulk operations: Content'
Under SELECTED BULK OPERATIONS choose 'Modify entity values' checkbox
Scroll down a little, you will see OPERATION SETTINGS choose that date Field you want to edit.
Click 'Apply' button
Click 'Save' to save that View


Go to the View Page you just created.


Under Operations choose 'Modify entity values' 
Select one or all of them (I suggest you do one as a test).
Click the 'Execute' button.
This page will allow you to set the Date to anything you want, even nothing. TIP! for Date if you want no date you need to change all values to blank. You can't delete just the hours, you need to set all of them to blank, which gives you the result you requested.
Click the 'Next' button.
Click the 'Confirm' Button.


Congrats. Go back and run it on all of them, if it went as expected on the one test node.