TrustManagerService: remove unused field

Remove the mVirtualDeviceManager field which is no longer used.  This
fixes an errorprone warning which showed up on an unrelated CL.

Ignore-AOSP-First: Fixes ag/25111900
Test: build
Bug: 300402201
Bug: 306620764
Change-Id: I189f6ff0def64a953ae75a0356661e41361d407b