unable to remove core snap on classic when using snapd snap

[Ian Johnson]

After launching a fresh xenial VM with snapd 2.40 installed from the archives, I am able to install the snapd snap and use snapd from there. However, after doing this, if I install a snap with `base: core`, then even after removing the snap that has `base: core`, I cannot remove the core snap:

```
ubuntu@busy-collie:~$ snap list
Name Version Rev Tracking Publisher Notes
core18 20200124 1668 stable canonical✓ base
snapd 2.42.5 5754 stable canonical✓ snapd
test-snapd-sh-core18 1.0 7 stable cachio -
ubuntu@busy-collie:~$ snap version
snap 2.42.5
snapd 2.42.5
series 16
ubuntu 16.04
kernel 4.4.0-173-generic
ubuntu@busy-collie:~$ sudo snap install hello-world
sudo snap install hello-world
hello-world 6.4 from Canonical✓ installed
ubuntu@busy-collie:~$ snap list
Name Version Rev Tracking Publisher Notes
core 16-2.42.5 8268 stable canonical✓ core
core18 20200124 1668 stable canonical✓ base
hello-world 6.4 29 stable canonical✓ -
snapd 2.42.5 5754 stable canonical✓ snapd
test-snapd-sh-core18 1.0 7 stable cachio -
ubuntu@busy-collie:~$ sudo snap remove hello-world
sudo snap remove hello-world
hello-world removed
ubuntu@busy-collie:~$ sudo snap remove core
error: cannot remove "core": snap "core" is not removable
```

I think if the system is using the snapd snap, we should be able to remove the core snap as a base, if no app snaps are using "core" as a base snap.