Remove UnixChannelFactory from preloaded-classes-blacklist

Initialization issues with FileDescriptor and UnixChannelFactory led
to UnixChannelFactory being put on the preloaded classes
blacklist. This was fixed in https://r.android.com/700445 and is no
longer required.

Bug: 109735195
Bug: 144502743
Test: art/test.py --host --prebuild --64 -r -t 071-dexfile-map-clean
Test: atest PreloadCheck
Change-Id: Idc9f4c5b39be06c0a77b092972cc2ffced37d4d1