So you want to delete the default Documents (or Shared Documents) library - which you should but in many site templates it cannot be deleted (the Delete this library link isn't shown).
Quick and easy, do it in PowerShell:
# Set Site URL, i.e. http://<site>, http://site/sites/abc, https://, etc.
$theWeb = Get-SPWeb <Site URL>
# Display the object (will display the URL to the site)
$list = $theWeb.Lists[“Documents”]
# Display List Details
# Make it deletable
$list.AllowDeletion = $True
# Update it
# Now delete it
PS: This is also a reminder for myself - I know what to do just forget sometimes!