Revert "Add VTS readback tests for buffer slot clearing"

This reverts commit d103cd68.

NOTE: This is not a full revert, it leaves the production code in place and only reverts the VTS test changes.

Reason for revert: b/262370410

Test: atest VtsHalGraphicsComposerV2_1TargetTest on Oriole
Test: atest VtsHalGraphicsComposerV2_2TargetTest on Oriole
Test: atest VtsHalGraphicsComposerV2_3TargetTest on Oriole
Test: atest VtsHalGraphicsComposerV2_4TargetTest on Oriole
Bug: 262370410
Change-Id: I336beb9f0ed81b305b26c6d8e9da70cfabe04bbd