Use regular std::filesystem

Now that std::filesystem is available there's no need to use
std::experimental::filesystem.

Note the '/' operator does behave differently when the RHS is an
absolute path (starts with /), but none of the  uses here make use of an
absolute path.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: Ice5ed7e85f6bf9c8355a66b5b00fe8550899a258
diff --git a/control/manager.cpp b/control/manager.cpp
index 8109367..adf9b53 100644
--- a/control/manager.cpp
+++ b/control/manager.cpp
@@ -29,7 +29,7 @@
 #include <xyz/openbmc_project/Common/error.hpp>
 
 #include <algorithm>
-#include <experimental/filesystem>
+#include <filesystem>
 
 namespace phosphor
 {
@@ -39,7 +39,7 @@
 {
 
 using namespace phosphor::logging;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 constexpr auto SYSTEMD_SERVICE = "org.freedesktop.systemd1";
 constexpr auto SYSTEMD_OBJ_PATH = "/org/freedesktop/systemd1";