public void delete(AccountIndex index) {
    mDatabase.delete(TABLE_NAME, whereClause(index), null);
    mDatabase.execSQL("VACUUM");
  }
Creates a SQL {@code WHERE} clause for an issuer.

@param issuer may be {@code null}
@return an appropriate SQLite {@code WHERE} clause for this {@code issuer}
java/com/google/android/apps/authenticator/otp/AccountDb.java