Revert "qrencode: Fix build dependency (libpng)"

This commit was not made by the package maintainer and needs to be reverted.
This reverts commit 1e2cd3318b.
Signed-off-by: Jonathan Bennett <jbennett@incomsystems.biz>

libs/qrencode/Makefile

@@ -21,8 +21,6 @@ PKG:FIXUP:=autoreconf
 
 include $(INCLUDE_DIR)/package.mk
 
-PKG_BUILD_DEPENDS:=libpng
-
 define Package/libqrencode
   SECTION:=libs
   CATEGORY:=Libraries