Use generator to enable ostree-remount.service and ostree-finalize-staged.path

We struggled for a long time with enablement of our "internal units",
trying to follow the philosophy that units should only be enabled
by explicit preset.

See https://bugzilla.redhat.com/show_bug.cgi?id=1451458
and https://github.com/coreos/rpm-ostree/pull/1482
etc.

And I just saw chat (RH internal on a proprietary system sadly) where
someone hit `ostree-remount.service` not being enabled in CentOS8.

Thinking about this more, I realized we've shipped a systemd generator
for a long time and while its only role until now was to generate `var.mount`,
but by using it to force on our internal units, we don't require
people to deal with presets anymore.

Basically we're inverting things so that "if ostree= is on the kernel
cmdline, then enable our units" and not "enable our units, but have
them use ConditionKernelCmdline=ostree to skip".

Drop the weird gyrations we were doing around `ostree-finalize-staged.path`
too; forking `systemctl start` is just asking for bugs.

So after this, hopefully we won't ever again have to think about
distribution presets and our units.

src/libostree/ostree-sysroot.c

@@ -190,7 +190,6 @@ ostree_sysroot_init (OstreeSysroot *self)
     { "mutable-deployments", OSTREE_SYSROOT_DEBUG_MUTABLE_DEPLOYMENTS },
     { "test-fifreeze", OSTREE_SYSROOT_DEBUG_TEST_FIFREEZE },
     { "no-xattrs", OSTREE_SYSROOT_DEBUG_NO_XATTRS },
-    { "test-staged-path", OSTREE_SYSROOT_DEBUG_TEST_STAGED_PATH },
     { "no-dtb", OSTREE_SYSROOT_DEBUG_TEST_NO_DTB },
   };