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
3 files changed