Removed PostProcess submodule

It's been trashed by a previous failed merge for some reason

.gitmodules

 [submodule "Assets/TestScenes/Big"]
 	path = Assets/TestScenes/Big
 	url = https://github.com/Unity-Technologies/ScriptableRenderLoopScenes
-[submodule "Assets/ScriptableRenderPipeline/PostProcessing"]
-	path = Assets/ScriptableRenderPipeline/PostProcessing
-	url = https://github.com/Unity-Technologies/PostProcessing
-	branch = v2
+