c++17: drop experimental::filesystem

Use the real filesystem library, and drop support for building with
experimental under c++14.

Change-Id: I47cbfc30b223db9dc28e9536ceb84e9fe3342e96
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/msl_verify.cpp b/msl_verify.cpp
index f80e179..36d4ffe 100644
--- a/msl_verify.cpp
+++ b/msl_verify.cpp
@@ -4,7 +4,7 @@
 
 #include <phosphor-logging/log.hpp>
 
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <regex>
 
@@ -15,7 +15,6 @@
 namespace image
 {
 
-namespace fs = std::experimental::filesystem;
 using namespace phosphor::logging;
 using AssociationList =
     std::vector<std::tuple<std::string, std::string, std::string>>;