Remove pointless assertion for magicBranch validation

The code here requires magicBranch.ctl to be defined if
magicBranch.baseCommit was filled in by the caller. Gerrit
is never run with -ea, so the assert is useless.

Remove the check, as the code will just helpfully NPE
when magicBranch.ctl.getRefName() is invoked.

Change-Id: I3cc30a089464ac613cbf69c2c486acf53467b009

gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java

@@ -1480,7 +1480,6 @@ public class ReceiveCommits {
         for (RevCommit c : magicBranch.baseCommit) {
           walk.markUninteresting(c);
         }
-        assert magicBranch.ctl != null;
         Ref targetRef = allRefs.get(magicBranch.ctl.getRefName());
         if (targetRef != null) {
           walk.markUninteresting(walk.parseCommit(targetRef.getObjectId()));