preloaded-classes-denylist: remove retired class

Remove com.android.server.BootReceiver$2 from the denylist as this
anonymous class was retired in change https://r.android.com/1565971.

Bug: 159164105
Bug: 179645293
Test: atest PreloadCheck
Change-Id: I94122241dc4cef84524ca83513a68d1fa8ffbe65