Reproduction steps:
1. Create new HDRP template project
2. Create a Decal Projector and a Decal Material (check note 1)
3. Assign the Material to Decal Projector and make sure the Decal is visible
4. Save and reload Scene
5. Change Size X/Y/Z on Decal Projector

Expected result: Decal size will change, there will be only one Decal
Actual result: ghost Decal is created and is left even after deleting GameObject with Decal Projector

Reproducible with: HDRP 7.1.6 (2019.3.0f3, 2020.2.16f1),
Not reproducible with: HDRP 4.10.0 (2018.4.14f1), HDRP 6.9.2 (2019.2.16f1), HDRP 7.1.2(2020.1.0a16)

Notes:
1. To test with HDRP 7.1.6 you might need to change Material Color to something else than white as Decal is invisible with default white Color
2. Issue only reproduces after Scene reload and changing Size property
3. Ghost Decal is removed when Material it was created from is destroyed
4. This is a Package regression