Replace std::experimental::filesystem with std::filesystem

Above c++17, std::filesystem is already supported, so replace
std::experimental::filesystem with std::filesystem.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I864bd80905e9b61428d17b20a91b3482198d20bc
diff --git a/Makefile.am b/Makefile.am
index 14cd3f7..560ba4b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -8,8 +8,7 @@
 
 phosphor_ledcontroller_LDFLAGS = $(SDBUSPLUS_LIBS) \
                                  $(SYSTEMD_LIBS) \
-                                 $(PHOSPHOR_DBUS_INTERFACES_LIBS) \
-				 -lstdc++fs
+                                 $(PHOSPHOR_DBUS_INTERFACES_LIBS)
 phosphor_ledcontroller_CFLAGS =  $(SDBUSPLUS_CFLAGS) \
                                  $(SYSTEMD_CFLAGS) \
                                  $(PHOSPHOR_DBUS_INTERFACES_CFLAGS)