graphics/vulkan-tools: drop unused CMake variable after 704278ed

https://github.com/KhronosGroup/Vulkan-Tools/commit/c4839afe758d

graphics/vulkan-tools/Makefile

@@ -18,8 +18,7 @@ USE_GITHUB=	yes
 GH_ACCOUNT=	KhronosGroup
 GH_PROJECT=	Vulkan-Tools
 CMAKE_OFF=	BUILD_CUBE BUILD_ICD
-CMAKE_ARGS=	-DGLSLANG_INSTALL_DIR:PATH="${LOCALBASE}" \
-		-DVULKAN_HEADERS_INSTALL_DIR:PATH="${LOCALBASE}"
+CMAKE_ARGS=	-DGLSLANG_INSTALL_DIR:PATH="${LOCALBASE}"
 
 OPTIONS_DEFINE=		WAYLAND XCB XLIB
 OPTIONS_DEFAULT=	WAYLAND XCB XLIB