Revert "Add VTS for new API for clearing buffer slots"

This reverts commit 78ff2d64.

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: Ica725cfaae6e2a08ed9c53eb9f17d6db1656bc93