Remove unused class ShadowKeyStore

Since the class android.security.KeyStore is going away, remove the
unused test utility class ShadowKeyStore that references it.

(ShadowKeyStore was originally used by a test, but
https://r.android.com/1353609 deleted that test.)

Bug: 326508120
Test: build
Flag: TEST_ONLY
Change-Id: I20f2330dee171a0bfe860a1318dc2f962a80bd19