Template Editor: Update "Delete template" to "Clear customizations" when appropriate

[jameskoster]

Related to #40881.

What problem does this address?

If you have customised a theme template, then open that template in the Template Editor, the UI suggests that template can be deleted (and the padding is messed up):

Clicking "Delete template" will clear your customisations but not delete the template.

What is your proposed solution?

Update the label to use the "Clear customizations" language from the Site Editor:

[Mamaduka]

I think this was mentioned in the original PR as well, but I forgot to do a follow-up.

I will assign this issue to myself and push an update next week.