Remove unused include. (facebook#19548)

Summary:
`LayoutableShadowNode.cpp` includes `"LayoutableShadowNode.h"` as well as `<fabric/core/LayoutContext.h>`. In turn, `LayoutContext.h` then includes `<fabric/core/LayoutableShadowNode.h>`. `LayoutContext.h` doesn't actually require `LayoutableShadowNode.h`, but this unnecessary inclusion can cause duplicate definition errors if the two include paths don't map to exactly the same file. This patch removes the unnecessary include.

The CI's build system should cover the testing needed.

[INTERNAL] [MINOR] [fabric] - Remove an unnecessary include in fabric/core/layout.
Closes facebook#19548

Differential Revision: D8313337

Pulled By: shergin

fbshipit-source-id: 2e01e29ff25131543d9a8601483c2e716c7437be

ReactCommon/fabric/core/layout/LayoutContext.h

@@ -7,9 +7,6 @@
 
 #pragma once
 
-#include <unordered_set>
-
-#include <fabric/core/LayoutableShadowNode.h>
 #include <fabric/graphics/Geometry.h>
 
 namespace facebook {