libpng: remove host build

Nothing uses this.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/libpng/Makefile

@@ -24,7 +24,6 @@ PKG_FIXUP:=autoreconf
 PKG_INSTALL:=1
 PKG_BUILD_PARALLEL:=1
 
-include $(INCLUDE_DIR)/host-build.mk
 include $(INCLUDE_DIR)/package.mk
 
 define Package/libpng
@@ -69,5 +68,4 @@ define Package/libpng/install
 	$(CP) $(PKG_INSTALL_DIR)/usr/lib/libpng.so $(1)/usr/lib/
 endef
 
-$(eval $(call HostBuild))
 $(eval $(call BuildPackage,libpng))