github/lock: Use reason "resolved", which is the default

ejona86 · ejona86 · commit 2653cf4d6b20 · 2018-09-21T16:32:53.000-07:00
What the "reason" means was explained in grpc#4862 (comment) , and it's probably fair to accept the default.
diff --git a/.github/lock.yml b/.github/lock.yml
@@ -1,3 +1,2 @@
 daysUntilLock: 365
 lockComment: false
-setLockReason: false
