SF: more fixes for DisplayTransactionCommitTest#processesVirtualDisplayAdded

Removes another annoying mock that was particularly fragile.

BYPASS_IGBP_IGBC_API_REASON=wb work

Bug: 340933138
Flag: com.android.graphics.surfaceflinger.flags.wb_virtualdisplay2
Test: DisplayTransactionCommitTest
Change-Id: I369def79b381d42e94abdb4b807af2d03a0699a5