replace sdbusplus::message::variant_ns with std

now that sdbusplus has fully moved to std::variant and sdbusplus's
variant is just an alias to std::variant, there is no need to use the
sdbusplus namespace anymore.

Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
Change-Id: Ifaa21f06ff73c4432260a8215bbbf7bb1e4357fe
7 files changed