chore: remove erroneous shutdown unit test

Removes the `shutdown` unit test from `TransactionalOutboxImplSpec`:
- It does not make sense, i.e. there are two items (which also
  happen to be the exact same instance) queued for processing, but
  then only one of them is reset to pending.
- There is a separate suite `OutboxShutdownSpec` that provides a range
  of unit tests specifically for the `shutdown` method and covers all
  possible paths.

[SW-587]

core/src/test/groovy/io/github/bluegroundltd/outbox/integration/TransactionalOutboxImplSpec.groovy

@@ -98,49 +98,6 @@ class TransactionalOutboxImplSpec extends Specification {
       0 * _
   }
 
-  def "Should delegate to the outbox store when shutdown is called and there are tasks that didn't start execution"() {
-    given:
-      def pendingItem = OutboxItemBuilder.make().withType(type).build()
-      def items = [pendingItem, pendingItem]
-      def now = Instant.now(clock)
-
-    when:
-      transactionalOutbox.monitor()
-
-    then:
-      1 * monitorLocksProvider.acquire()
-      1 * store.fetch(_) >> { OutboxFilter filter ->
-        with(filter) {
-          outboxPendingFilter.nextRunLessThan == now
-        }
-        items
-      }
-      items.size() * store.update(_) >> { OutboxItem item ->
-        with(item) {
-          it.status == OutboxStatus.RUNNING
-          it.lastExecution == now
-          it.rerunAfter == item.lastExecution + DURATION_ONE_HOUR
-        }
-        return item
-      }
-      1 * monitorLocksProvider.release()
-      0 * _
-
-    when:
-      transactionalOutbox.shutdown()
-
-    then:
-      1 * store.update(_) >> { OutboxItem item ->
-        with(item) {
-          it.status == OutboxStatus.PENDING
-          it.nextRun == now
-          it.rerunAfter == null
-        }
-        return item
-      }
-      0 * _
-  }
-
   def "Should delegate to outbox store when `cleanup` is invoked"() {
     given:
       def now = Instant.now(clock)