Use std::variant

When first committed, I had a misunderstanding of what dedup variant
does.  In this case, because we unpack all variants of 64/32 bit and int
vs uint, so in this case, dedup variant doesn't do anything, and just
adds template complexity.  In terms of developer readability,
std::variant is a c++ concept that is infinitely googlable, and not
having to look it up through sdbusplus should be better.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ia73b642fd06b24187503196bcdfecb9c896d62c9