Revert "Add a notion of "physical" sysroot, use for remote writing"

This reverts commit 1eff3e8343. There
are a few issues with it.  It's not a critical thing for now, so
let's ugly up the git history and revisit when we have time to
debug it and add more tests.

Besides the below issue, I noticed that the simple `ostree remote add`
now writes to `/ostree/repo/config` because we *aren't* using the
`--sysroot` argument.

Closes: https://github.com/ostreedev/ostree/issues/901

Closes: #902
Approved by: mike-nguyen

src/libostree/ostree-repo-private.h

@@ -93,7 +93,6 @@ struct OstreeRepo {
   int objects_dir_fd;
   int uncompressed_objects_dir_fd;
   GFile *sysroot_dir;
-  GWeakRef sysroot; /* Weak to avoid a circular ref; see also `is_system` */
   char *remotes_config_dir;
 
   GHashTable *txn_refs;