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/file.hpp b/file.hpp
index 34b1422..2819791 100644
--- a/file.hpp
+++ b/file.hpp
@@ -1,14 +1,12 @@
 #pragma once
 
 #include <stdio.h>
-#include <experimental/filesystem>
+#include <filesystem>
 namespace phosphor
 {
 namespace user
 {
 
-namespace fs = std::experimental::filesystem;
-
 /** @class File
  *  @brief Responsible for handling file pointer
  *  Needed by putspent(3)
@@ -71,9 +69,9 @@
         }
 
         // Needed for exception safety
-        if (removeOnExit && fs::exists(name))
+        if (removeOnExit && std::filesystem::exists(name))
         {
-            fs::remove(name);
+            std::filesystem::remove(name);
         }
     }