Don't crash if we get a onRemove() callback from Fingerprint

Before this fix, it was possible to get a callback after the activity
finishes.  This just ensures the operation is a no-op.

Fixes bug 37106041

Test: remove all but last fingerprint and hit back while rotating; check that
Settings doesn't crash.

Change-Id: I99df4f7c06e345d1ed6041a75606b0cc43edb300