Don't clean the sleep queue

The code was there to prevent sleep timeouts from leaking to next tests
and causing timeouts in enabling rollbacks.

However, testEnableRollbackTimeoutFailsRollback_MultiPackage becomes
flaky due to the clean-up:

1. Install#commit posts SessionCallback#onFinished to the handler thread
   of RollbackManagerServiceImpl. mSleepDuration.clear() is called
   there.
2. RollbackManager#blockRollbackManager posts mSleepDuration.addLast()
   to the handler thread.
3. Depending on the thread scheduling, mSleepDuration.clear() might
   happen after mSleepDuration.addLast() even though Install#commit
   happens before RollbackManager#blockRollbackManager.
4. This nullifies the effect of #blockRollbackManager and fails the test
   because enable-rollback-timeout didn't work as intended.
5. See b/145641719 comment#2 for detailed explanation.

We remove the code to prevent the above scenario from happening at the
risk of leaking sleep timeouts to next tests. However, this hypothetical
leaking never happens. We should be fine removing the code.

If the leak does happen, it could mean a bug in the install flow of the
package manager. We need much deeper analysis before fixing the problem.

Bug: 145641719
Test: atest RollbackTest
Change-Id: I37dff9b66e69578749eaff332b84879c7956b911