drm_hwcomposer: Remove SquashState class declaration

There is no use of SquashState class, so the declaration could be removed
in the scope of cleanup.

Signed-off-by: Matvii Zorin <matvii.zorin@globallogic.com>
diff --git a/compositor/DrmDisplayComposition.h b/compositor/DrmDisplayComposition.h
index 5d076b3..6e8b5d2 100644
--- a/compositor/DrmDisplayComposition.h
+++ b/compositor/DrmDisplayComposition.h
@@ -31,7 +31,6 @@
 
 class Importer;
 class Planner;
-class SquashState;
 
 enum DrmCompositionType {
   DRM_COMPOSITION_TYPE_EMPTY,