core: make "tmpfs" dependencies on swapfs a "default" dep, not an "implicit"

There should be a way to turn this logic of, and DefaultDependencies=
appears to be the right option for that, hence let's downgrade this
dependency type from "implicit" to "default, and thus honour
DefaultDependencies=.

This also drops mount_get_fstype() as we only have a single user needing
this now.

A follow-up for #7076.