Revert "Don't mount /nix/store ro if it's a mountpoint"

Now that nixUnstable supports remounting in the "/nix/store is a
mountpoint" case, this is no longer necessary.

This reverts commit f1d48aec43947e5dddae8ba02e3133ee8d0d0ff9.

Signed-off-by: Shea Levy <shea@shealevy.com>