remote prune: warn dangling symrefs

remote prune: warn dangling symrefs

If you prune from the remote "frotz" that deleted the ref your tracking
branch remotes/frotz/HEAD points at, the symbolic ref will become
dangling.  We used to detect this as an error condition and issued a
message every time refs are enumerated.

This stops the error message, but moves the warning to "remote prune".

Signed-off-by: Junio C Hamano <gitster@pobox.com>