Remove second initialization of writech in Open (#1382)

The `db.writeCh` is being initialized for the second time in `db.Open` which
causes a race condition. The write channel was being closed in the `doWrites`
goroutine earlier but that code was removed in
2b39009.
We no longer need the second initialization. See PR #1382 for full stack trace.

db.go

@@ -399,7 +399,6 @@ func Open(opt Options) (db *DB, err error) {
 	db.orc.readMark.Done(db.orc.nextTxnTs)
 	db.orc.incrementNextTs()
 
-	db.writeCh = make(chan *request, kvWriteChCapacity)
 	db.closers.writes = y.NewCloser(1)
 	go db.doWrites(db.closers.writes)