c++17: drop experimental::filesystem

Use std::filesystem, and drop support for building with experimental
under c++14.

Tested: Build the repo.
Change-Id: I4af0d9c034dbfef5a65153ba5447b86c961aebf1
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/users.cpp b/users.cpp
index 562a0ad..1dbe85a 100644
--- a/users.cpp
+++ b/users.cpp
@@ -14,6 +14,7 @@
 // limitations under the License.
 */
 
+#include <filesystem>
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/wait.h>
@@ -56,8 +57,7 @@
              std::vector<std::string> groups, std::string priv, bool enabled,
              UserMgr &parent) :
     Interfaces(bus, path, true),
-    userName(std::experimental::filesystem::path(path).filename()),
-    manager(parent)
+    userName(std::filesystem::path(path).filename()), manager(parent)
 {
     UsersIface::userPrivilege(priv, true);
     UsersIface::userGroups(groups, true);