Remove stray get() call in ChangeControl#isAssignee

When 138aec1244 (Assignee in ReviewDb,
2016-09-23) converted currentAssignee from an Optional<Account.Id>
to an Account.Id, it forgot to remove this get() call.  As a result
the method always returns false.

Noticed by error-prone
(http://errorprone.info/bugpattern/EqualsIncompatibleType).

Change-Id: I13c778c7e1afdcb70c2790da12f770c87d8197ec

gerrit-server/src/main/java/com/google/gerrit/server/project/ChangeControl.java

@@ -370,7 +370,7 @@ public class ChangeControl {
     Account.Id currentAssignee = notes.getChange().getAssignee();
     if (currentAssignee != null && getUser().isIdentifiedUser()) {
       Account.Id id = getUser().getAccountId();
-      return id.equals(currentAssignee.get());
+      return id.equals(currentAssignee);
     }
     return false;
   }