HdyComboRow: Don't use g_autoptr for GEnumClass

g_autoptr for GEnumClass was added post 2.56, so using it makes it
harder for people to package for distros. Not using g_autoptr there
doesn't make the code much less readable.
4 jobs for g-enum-no-auto-ptr in 3 minutes and 10 seconds (queued for 1 second)