device: Delete existing print in EnrollStart

gnome-control-center expects to be able to re-enroll an existing print
when calling EnrollStart without deleting it first. As such, implicitly
delete the existing print rather than throwing an error.

Ideally, we'll change the API, but we need to give API users time to
adjust to the world.

tests/fprintd.py

@@ -2315,9 +2315,21 @@ class FPrintdVirtualDeviceEnrollTests(FPrintdVirtualDeviceBaseTest):
     def test_enroll_already_enrolled_finger(self):
         self.enroll_image('whorl', start=False)
 
+        # We can enroll a new image deleting the first
+        self.device.EnrollStart('(s)', 'left-middle-finger')
+        self.enroll_image('arch', start=False)
         self.stop_on_teardown = False
-        with self.assertFprintError('FingerAlreadyEnrolled'):
-            self.device.EnrollStart('(s)', 'left-middle-finger')
+
+        # If we verify, 'arch' will match, 'whorl' will not match
+        self.device.VerifyStart('(s)', 'any')
+        self.send_image('whorl')
+        self.assertVerifyNoMatch()
+        self.device.VerifyStop()
+
+        self.device.VerifyStart('(s)', 'any')
+        self.send_image('arch')
+        self.assertVerifyMatch()
+        self.device.VerifyStop()
 
     def test_enroll_duplicate_image(self):
         self.enroll_image('whorl', finger='left-thumb', start=False)