Delete nextContributorId sequence

This sequence has never been used. I manually bumped it twice when
setting up the review.source.android.com site and then realized
how useless it was, as I could have assigned the indexes 1 and 2
by hand when creating the site.

Remove the sequence to clean up the schema.

Change-Id: I0408b1ab25ce1b6e0816303aa363544876b541a2

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/ReviewDb.java

@@ -120,10 +120,6 @@ public interface ReviewDb extends Schema {
   @Sequence(startWith = 1000000)
   int nextAccountId() throws OrmException;
 
-  /** Create the next unique id for a {@link ContributorAgreement}. */
-  @Sequence
-  int nextContributorAgreementId() throws OrmException;
-
   /** Next unique id for a {@link AccountGroup}. */
   @Sequence
   int nextAccountGroupId() throws OrmException;