bug#44306: package-delete exiting on encountering system/dependency packages

[Boruch Baum]

When attempting to perform package-autoremove to clean up obsolete
packages, my emacs would abort with an error that `foo' is a system
package. Thus, it had become impossible to perform the operation. I
altered function package-delete to replace its two calls to function
`error' with simple `message' and can now clean up the packages.

I'm now left with function package-autoremove reporting:

12 packages will be deleted:
session, muttrc-mode, markdown-toc, htmlize, diminish, dictionary,
debian-el, csv-mode, browse-kill-ring, boxquote, bm, magit-popup,
proceed? (y or n) y

All of which are 'system' packages (at least in debian) so they won't
actually be deleted by my hack, but at least the other ~40 were finally
able to be removed.

I have reservations that this hack is a true and correct solution for
the bug because I don't know enough about the 'package package, but it
does seem to do the job...

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