Remove android.security.KeyStore#getInstance()

This method no longer has any callers, so remove it.

(As with other methods in android.security.KeyStore, this did have
@UnsupportedAppUsage.  However, there is no reason for apps to use this.
Therefore, removing it is allowed by the non-SDK interface policy.)

Bug: 326508120
Test: Build
Change-Id: I76facc1d54bf656188d1d4c5745224fc49836132