Field timestamp default can not be set to none.

Problem/Motivation

If I create a field timestamp that mistakenly had a value set for default, it is not possible to change it to have no default because the seconds value which is locked causes the field setting to pick up a new timestamp that is the time of save of the settings

Steps to reproduce

1. Add a timestamp field to a content type, include a default
2. Save the configuration
3. Edit the field
4. Delete the date and time fields from the default
5. Notice the seconds is disabled which prevents deletion of the seconds
6. Save the config
7. Edit again and notice that you have new default that matches the time when you pressed save.

Proposed resolution

If date is blank, blank the times as well. Can't have a timestamp with no date.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet